This manual discusses the troubleshooting and overhaul of an engine. You first learn about horsepower, including a brief discussion on dynamometers and their role in engine overhaul.
Other topics include graphs and diagrams, engine troubleshooting, valve maintenance, servicing of cylinder heads, crankshafts, cylinders, and pistons and rings, and the operational testing of the engine after the overhaul has been completed with pre-startup, initial startup and run-in.
As a supervisor, your expertise will be invaluable in the maintenance of automotive and heavy construction equipment .
When you have completed this manual, you will be able to:
Whenever someone mentions horsepower, the first thing that comes to mind is cars-- fast cars with extreme torque, vehicles with large power and extreme speed. But what makes horsepower measuring so meaningful? And why is it called horsepower?
Engines are rated according to horsepower and torque. The speed and fuel setting of an engine will determine its actual rated horsepower setting. Horsepower is the ability to maintain a load at a desirable or fixed speed. It is a measure of how fast work can be done. Horsepower is produced as a direct result of combustion in the cylinder.
Combustion pressures force the piston down the cylinder, producing work. The amount of work produced on the power stroke can be determined using the following formula:
Work = Force x Distance
- Force = Pressure acting on the piston in psi
- Distance = Length of piston stroke
The more fuel released into the cylinder during a given period, the greater the amount of horsepower produced.
James Watt. The word horsepower was created in 1782 by an engineer named James Watt while he was developing a way to improve the power of a steam engine (Figure 1). While watching horses haul coal out of a coal mine, he came up with the idea of defining the power exerted by these animals. He found that, on average, a mine pony could do 22,000 foot-pounds of work in a minute. He then increased that number by 50 percent and pegged the measurement of horsepower at 33,000 foot- pounds of work in one minute.
Figure 1 - Portrait of James Watt (1736-1819).
What horsepower means is this: In Watt's judgment, one horse can do 33,000 foot-pounds of work every minute (Figure 2). So, imagine a horse raising coal out of a coal mine as shown in the illustration. A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute or 33 pounds of coal1,000 feet in one minute, or 1,000 pounds 33 feet in one minute. You can make up whatever combination of feet and pounds you like. As long as the product is 33,000 foot-pounds in one minute, you have a horsepower.
Figure 2 - Illustration showing how James Watt determined horsepower.
Watt used his new found term to rate the power of the steam engine, (Figure 3). Since most people were unfamiliar with the steam engine, he had to come up with a comparison measurement that the normal farmer of the day would understand. The draft horse was widely used enough that most people of the time had at least an idea of the animal's capability.
Figure 3 - 1769 - James Watt's improved steam engine powered the industrial revolution.
As with any measurement there are different variations and different methods of measuring horsepower, or hp. The normal measurement of horsepower is called mechanical horsepower.
|1 HP =||33,000 ft/lb.|
Energy. Energy is best defined as the capacity for producing work. Energy exists in a number of forms; kinetic, potential, electrical, thermal, chemical, and nuclear are its common forms.
The first law of thermodynamics states that energy can neither be created nor destroyed; however, the way in which energy manifests itself can be changed. In an internal combustion engine, the potential heat energy of a fuel is released when it is combusted, producing pressure that acts on a piston and drives it through its power stroke producing kinetic energy. Not all of the potential heat energy of the fuel can be successfully converted to kinetic energy. This energy is described as rejected heat and must be dissipated to the atmosphere.
Effects of Heat Energy. Heat is easily converted into mechanical energy. The sun's heat daily raises tons of water vapor high into the atmosphere so all the mechanical energy of falling water, whether as rain, in rivers, or in glaciers, stems directly from the sun's heat. In the heated engine, the heat released from the burning of fuel is converted into mechanical or kinetic energy.
Force. Force is generally defined in terms of the effects it produces, although it should always be remembered that forces can be exerted with no result. If force is applied to a body at rest, it may be sufficient to cause the body to move; however, it might not. Using the standard system, force is measured in pounds.
In a diesel engine, force is represented by cylinder pressure. This force acts on the sectional area of the piston crown and is transmitted to rotary motion by acting on the crank throw. The amount of force is controlled by the amount of fuel delivered to the engine cylinder because of the excess air factor in diesel.
Work. Work is accomplished when force produces a result. When the definition is applied to an engine, work is accomplished when force acting on the piston results in piston travel. In the following formula, standard values are always listed first:
Work = Force x Distance
- Work is expressed in watt-seconds
- Force is expressed in pounds
- Distance is expressed in feet
To use a non-engine analogy to define work, if two persons of identical weight run exactly 100 yards, each has accomplished the same amount of work
Torque. Torque is turning effort. In a typical engine, the force acting on the piston is transmitted to a crankshaft throw. The throw is a lever. Torque is the product of force on the torque arm or crank throw and its perpendicular distance from the shaft center. The greater the distance of the throw centerline from the crankshaft main centerline, the greater the leverage and the more potential torque. The ability to produce torque in an engine is directly related to its cylinder pressures. For instance, peak torque will always occur when cylinder pressures peak.
For example: A bicycle has a crankshaft driven by a pair of throws called pedals, offset 180 degrees. Peak torque is at a maximum whenever muscle force acting on the pedals is at a maximum. High torque is required to propel a bicycle and the weight of its rider up a steep hill so a gear selection must be made so that the required torque is within the capabilities of its engine, the cyclist.
Indicated horsepower is the power transmitted to the pistons from the expanding combustion gases. Indicated horsepower is not measured with a test instrument, but is mathematically calculated. Indicated horsepower does not take into account frictional power losses of moving parts or the power losses by accessory items such as water pumps, blowers, oil pumps, fuel injection pumps, and cooling fans.
The formula for calculating engine indicated horsepower is:
|Indicated Horsepower =||P x L x A x N x C.|
- P = mean indicated Pressure in psi
- L = Length of stroke of the piston in feet
- A = Piston cross-sectional Area
- N = Number of power strokes per cylinder per minute
- C = Number of engine Cylinders
The PLANC formula requires the size of the piston to be expressed in terms of its area. The size of the piston, however, is most often expressed in terms of its diameter -- not it's area. In order to use the PLANC formula, you must convert the given piston diameter (d) to area (A).
Recall from your plane geometry that the area of a circle (the area of the piston in this case) is given by:
A = pr2
But we are given the diameter (d) of the piston, and radius. This is easily handled by recalling that the radius of a circle is one-half its diameter:
r = ½d
By substituting r = ½d into A = pr2, we get:
A = p(½d)2
This is the power measured at the flywheel of an engine and is the actual amount of power that can be delivered at a certain speed with a wide-open throttle. It is always lower than indicated horsepower (calculated power) because of the amount of power consumed by the engine in overcoming internal friction and pumping losses-usually about 10% to 20% less than indicated horsepower for the same engine. The term is used to describe actual engine power over calculated power and is derived from the braking device, usually a dynamometer, that is applied to measure the horsepower the engine develops.
A dynamometer is an instrument designed primarily to measure power. Essentially, it applies turning resistance to the torque output (twisting effort) to another machine and accurately measures the applied resistance. Power is the rate of accomplishing work. When power is tested on a dynamometer, its factors are torque and time. The torque output of an engine is accurately measured by the dynamometer and factored with time (rpm) to calculate its power. Most dynamometers are electronic, and will take care of the math and display the power in units of brake horsepower (bhp) or kilowatts (kW).
There are two general categories of dynamometers, and they are defined by the method used to apply a resistance to the turning effort of the engine or chassis being tested. An electromotive dynamometer is basically an electric motor turned in reverse. The engine is coupled to the dynamometer armature and rotates it. As current is switched to flow through the induction coils of the electromotive dynamometer, resistance to the turning effort of the engine increases. The more current flowed through the dynamometer coils, the stronger the electromagnetic field produced and the greater the amount of torque required by the engine. The second category of dynamometer is hydraulic. The hydraulic medium used is usually water, but other types of hydraulic media are also used. The critical component in the hydraulic dynamometer is a load cell or multiple load cells. These use a principle similar to that used by a hydraulic driveline retarder such as the Caterpillar brake saver, where water is flowed through the cell and acts on an impeller. Inlet and outlet valves are used to control the flow of the hydraulic medium into and out of the load cell and define the torque required to rotate the impeller. All dynamometers measure torque. Most also measure rotational speed. When both torque and rpm are known, brake horsepower can be calculated using the equations discussed previously.
When performing chassis dynamometer testing of diesel engines whether for purposes of diagnosis or for performing an engine run-in routine, all of the gauges and instrumentation should be connected to the engine being tested. Loading an engine down on a dynamometer is the ultimate performance test for an engine, and it should be as thoroughly monitored as possible. It takes a little extra time to connect all the required gauges and instruments, but they will help ensure accurate diagnoses and complete reports. Electronic printouts do not look professional when half the data categories are left blank.
An engine dynamometer is a piece of test equipment that enables the mechanic to fully load-test the overhauled engine before it is reinstalled in the vehicle/equipment (Figure 4).
Figure 4 - Water brake engine dynamometer.
With an engine dynamometer, the engine can be fully checked and fine-tuned before it is placed into service. The engine is mounted in the stand, and fuel lines, cooling system hoses, exhaust system, oil lines, and all available instrumentation connected. A typical dynamometer will include the following gauges and meters for monitoring run-in conditions:
Most dynamometers will have one gauge upstream and one downstream of the air cooler to indicate how well it is doing the job.
Be particularly observant during any run-in procedure to detect possible problems that may develop. Monitor all gauges and readings on the dynamometer, vehicle dash display, or engine control panel. Look for the following problems:
If the engine develops any of these problems, it must be immediately shut down. Investigate and correct any problem before continuing the test.
A chassis dynamometer allows for the controlled vehicle run-in without the need of a road test (Figure 5). The vehicle's drive axles are placed on a set of in-ground rollers and operated through a specific range of speeds and loads to monitor performance. Ensure the drive axles are properly aligned and in contact with the rollers, and then chain them into position.
Figure 5 - A chassis dynamometer
A chassis dynamometer must be properly calibrated. On vehicles with bogie axles, the dynamometer must be adjusted so there is no more than a 5 mph difference between axle speeds during testing. On vehicles equipped with a power divider lockout control, the lockout should be engaged during testing. Vehicles without lockout control may require a specially fabricated lockout device. Finally, it may be necessary to disconnect the front propeller shaft from the transfer case. Check the engine manual for these and other precautions prior to testing.
Friction horsepower is the difference between indicated horsepower and brake horsepower. Actually, it is the power required to overcome friction within the engine, such as friction between engine parts, resistance in driving accessories, and loss due to pumping action of the pistons. The latter maybe compared to the effort required to raise the handle of a hand-operated tire pump. It may be difficult to define friction horsepower properly, but with proper maintenance, it can be reduced to improve the mechanical efficiency of the engine.
When speaking of horsepower, generally it is referred to as horsepower at the wheels. There are power losses through the drive train, etc., so wheel horsepower is always lower than flywheel horsepower. A dynamometer will attempt to calculate the losses of the vehicle/equipment caused by friction of the gears, bearings, drivetrain, etc. to obtain power at the flywheel. The flywheel horsepower (engine horsepower) is not 100% accurate, as the only way to get the exact flywheel horsepower is to remove the engine from the vehicle/equipment and bolt it to an engine dynamometer in a test cell. Brake horsepower measures the power being put to the ground, while flywheel horsepower measures the power the engine produces.
The engine of any piece of equipment is taken for granted as long as it runs smoothly and efficiently. Eventually, however, in the manual of normal operation engines stop operating efficiently or entirely. When this happens, the mechanic must be able to determine the cause and know what is needed to correct the problem.
Generally, the supervisor does not perform engine repairs, but due to your extensive experience in troubleshooting and overhauling, as supervisor you are responsible for seeing that these repairs are performed correctly and efficiently by assisting and instructing those performing the work.
Since you might encounter several different models of internal combustion engines, both gasoline and diesel, for automotive and heavy construction equipment, it is impractical to specify any detailed troubleshooting or overhaul procedures for all the engines within the inventory. However, here are some elementary principles that apply to all engine troubleshooting and overhauls:
Since maintenance cards, manufacturers' technical manuals, and various instructions contain repair procedures in detail, this manual will be limited to general information on some of the troubles encountered during overhaul, their causes, and methods of repair.
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Graphs and diagrams are abbreviated methods of recording operational and maintenance data.
Manufacturers' operational and maintenance manuals often contain graphs and diagrams. The technical bulletins, prepared chiefly for tune-up mechanics, may use a particular graph or diagram to eliminate pages of written description that otherwise would be necessary.
Figures 4-6 and 4-7 are examples of graphs that describe engine performance in terms of brake horsepower and fuel consumption. Dynamometer tests provide the data used in plotting the performance curves for each engine. Figure 8 is another example of a graph. It shows that the amount of torque an engine produces varies with its speed. The relationship between torque and horsepower is shown in Figure 9.
Figure 6 - Performance curves of a typical six-cylinder gasoline engine.
Horsepower is related to both torque and speed. When both are increasing, as they do between 1,200 and 1,600 rpm, then horsepower goes up sharply. As torque reaches maximum and begins to taper off, horsepower continues to rise to maximum. The horsepower starts to decline beyond rated speed where torque falls off sharply.
Figure 7 - Performance curves of a typical six-cylinder diesel engine.
Figure 8 - Graph showing relationship between torque and speed.
Figure 9 - Graph indicating the relationship between torque and horsepower.
Engine timing is largely a matter of opening and closing valves or ports and of adjusting ignition or fuel injection so that these events occur at the proper time in the cycle of engine operation. Timing diagrams depict these events in relation to each other and in relation to top dead center (TDC) and bottom dead center (BDC). They are useful to the mechanic for quick and easy reference. However, before timing diagrams can be useful, you must recall a few facts about engine cycles.
The four-stroke-cycle engine makes two complete crankshaft revolutions in one cycle (intake, compression, power, and exhaust). The two-stroke-cycle engine completes a cycle with just one crankshaft revolution. With diesel engine cycles (two- and four- stroke), the event of fuel injection will be shown on the timing diagram instead of spark ignition, which is common to gasoline engine operating cycles.
Figure 10 shows a typical timing diagram for a four-stroke-cycle diesel engine. The actual length of the strokes shown and the beginning of fuel injection will vary a few degrees in either direction, depending on the specific manufacturer's recommendations. Follow the events in this cycle by tracing the circular pattern around two complete revolutions in a clockwise direction. Start TDC with the beginning of the POWERSTROKE. Compression is at its peak when fuel injection has been completed and combustion is taking place. Power is delivered to the crankshaft as the piston is driven downward by the expanding gases in the cylinder. Power delivery ends when the exhaust valve opens.
After the exhaust valve opens, the piston continues downward to BDC and then upward in the EXHAUST STROKE. The exhaust gases are pushed out of the cylinder as the piston rises to TDC, and the exhaust valve closes a few degrees after TDC to ensure proper scavenging. The crankshaft has made a complete revolution during the power and exhaust strokes.
The intake valve opens a few degrees before TDC near the end of the upward exhaust stroke to aid in scavenging the cylinder. As the crankshaft continues to rotate past TDC, the INTAKE STROKE begins. The intake stroke continues for the whole downward stroke and part of the next upward stroke to take advantage of the inertia of the incoming charge of fresh air.
Figure 10 - Typical timing diagram of a four-stroke-cycle diesel engine.
The rest of the upward stroke is the COMPRESSION STROKE, which begins at the instant of intake valve closing and ends at TDC. FUEL INJECTION may begin as much as 40° before TDC and continue to TDC, thus completing the power cycle and the second complete revolution of the engine.
By showing an approximate ignition point in place of fuel injection, Figure 10 could easily represent a timing diagram for a typical gasoline engine.
For additional information on diesel fuel injection system tests that can be made both in the shop and in the field, refer to the manufacturer's service manual.
Figure 11 shows a timing diagram of a two-stroke-cycle diesel engine. This engine is typical of the General Motors series, which uses a blower to send fresh air into the cylinder and to clear out the exhaust gases. The movement of the piston itself does practically none of the work of intake and exhaust, as it does in a four-stroke-cycle engine. This fact is important to the mechanic in detecting two-stroke-cycle diesel engine power losses.
Figure 11 - Timing diagram of a two-stroke-cycle diesel engine.
Beginning at TDC in Figure 11, the fuel has been injected, and combustion is taking place. The piston is driven down, and the power is delivered to the crankshaft until the piston is just a little more than halfway down. The exhaust valves (two in each cylinder) open 92 1/2° after TDC. The exhaust gases blow out through the manifold, and the cylinder pressure drops off rapidly.
At 132° after TDC (48° before BDC), the intake ports are uncovered by the downward movement of the piston. Scavenging air under blower pressure swirls upward through the cylinder and clears the cylinder of exhaust gases. This flow of cool air also helps to cool the cylinder and the exhaust valves. Scavenging continues until the piston reaches 44 1/2° after BDC. At this point, the exhaust valves are closed. The blower continues to send fresh air into the cylinder for just a short time (only 3 1/2° of rotation), but it is sufficient to give a slight supercharging effect.
The intake ports are closed at 48° after BDC, and compression takes place during the remainder of the upward stroke of the piston. Injection begins at about 22 1/2° before TDC and ends about 5° before TDC, depending on the engine speed and load.
The whole cycle is completed in one revolution of the crankshaft, and the piston is ready to deliver the next power stroke.
Theoretically, the power stroke of a piston continues for 180° of crankshaft rotation on a four-stroke-cycle engine. Best results can be obtained, however, if the exhaust valves are opened when the power stroke has completed about four-fifths of its travel. Therefore, the period that power is delivered during 720° of crankshaft rotation, or one four-stroke cycle, will be 145° multiplied by the number of cylinders in the engine. This may vary slightly according to the manufacturers' specifications. If an engine has two cylinders, power will be transmitted for 290° of the 720° necessary to complete the four events of the cycle. The momentum of the flywheel rotates the crankshaft for the remaining 430° of travel.
As cylinders are added to an engine, each one must complete the four steps of the cycle during two revolutions of the crankshaft. The number of power impulses for each revolution also increases, producing smoother operation. If there are more than four cylinders, the power strokes overlap, as shown in Figure 12. The length of overlap increases with the number of cylinders. The diagram for the six-cylinder engine shows a new power stroke starting each 120° of crankshaft rotation and lasting 145°. This provides an overlap of 25°. In the eight-cylinder engine, a power stroke starts every 90° and continues for 145°, resulting in a 55° overlap of power. Because the cylinders fire at regular intervals of firing order, this process will apply to either in-line or V-type engines.
Figure 12 - Power strokes in one-, four-, six-, and eight-cylinder engines.
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Power failures can result from minor troubles, such as loose or bare wires and disconnected or damaged fuel lines. When reported by the operator, these troubles are easy to detect without too much checking and testing. The supervisor must, however, make the mechanics aware that there probably was, in addition, an actual or contributing cause to the power failure. The supervisor must train the mechanics to look for this cause while making repairs. Unless eliminated, this may be the cause of major trouble later on.
Too often, problems associated with power loss occur within the engine and are not easily identified. It is these hard-to-find problems, with little or no visual indication, that keep the shop mechanics busy. An operator may notice an apparent power loss in the equipment and, because there is excessive smoke coming from the exhaust, report the trouble as improper carburetion, or, in the case of a diesel engine, as injector trouble. A less experienced mechanic may notice an increased engine temperature in addition to the exhaust smoke and diagnose the loss of power as improper valve action or as trouble in the cooling system. The diagnoses are comparatively simple through visual indications. But, as a senior mechanic, you know that there are many causes of power loss that have little or no visual indications. Examples are incorrect ignition timing, faulty coil or condenser, defective mechanical, vacuum or electronic spark advance, worn distributor cam, or slipping clutch. Any of them can cause a power loss.
After a deficiency has been located in an engine, it is usually easy to make the necessary corrections to eliminate the conditions causing the deficiency. Careful analysis and straight thinking, however, are often needed to find the cause of engine deficiencies. If a supervisor has a thorough knowledge of the basic engineering and operating principles of an engine, his or her job of training the mechanics will be easier and more interesting. In diagnosing engine deficiencies, the supervisor must never jump to conclusions and make a decision on the nature of repairs to be made before being sure that what will be done will eliminate the trouble. The mechanics must be able to interpret the engine instrument indications as well as use the proper testing devices.
Furthermore, they must be able to road test the equipment to determine whether repairs have been made satisfactorily and whether a part or several parts should be adjusted or replaced. Besides, the mechanic must know when and how to make emergency adjustments for every unit on the engine.
It may seem that some of the qualifications required of a good mechanic point to the know-how of an automotive engineer. However, no one person can know all about engines and also be an expert in repairing all types of powered equipment used in the Naval Construction Force. For instance, if the checks or instrument tests indicate some internal trouble in a magneto, carburetor, or fuel injection unit, the repairs should be made by mechanics who have experience or have been specially trained to use the equipment to do the particular job at hand. It is the supervisor who will be expected to have the answers to all the questions asked by less experienced mechanics.
The three basic factors that affect an internal combustion engine's power are as follows:
In the diesel engine, fuel is injected into each cylinder, and ignition depends on the heat of compression; in the gasoline engine, ignition and carburetion are independent. In both engines, of manual, proper action and timing of all three factors are necessary for the engine to produce its rated power.
An engine will run and develop its rated power only if all of its parts function or operate as they should. If any of these parts wear or break, requiring replacement or adjustment, the performance charts and engine specifications are "tools" that will help the mechanic bring those parts back to their original relationship to each other.
There are more factors NOT directly associated with engine working parts that must be considered in correcting engine power losses.
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As a general rule when troubleshooting a system complaint, investigate the possible causes that can be eliminated easily and inexpensively before proceeding to those that require more time. Avoid component disassembly until you have exhausted all other investigative options. This practice will help avoid presenting an inflated repair for a simple problem. It makes sense to develop some procedures for troubleshooting engine problems in the mechanic shops. A set of procedures will help the mechanics manage their troubleshooting procedures. Engine OEMs produce some excellent troubleshooting charts for their own products.
The troubleshooting approach used by the mechanics will depend on whether the engine is hydromechanically or electronically managed, and on the engine OEM. Keep in mind that the troubleshooting procedure required of most electronically managed engines is structured and sequential and must be adhered to. The following sections discuss some typical engine problems along with their possible causes and solutions, with the focus on hydromechanical problems.
Excessive oil consumption would probably first be noted by the operator who has to add oil to maintain the proper oil level. Two main causes of excessive oil consumption are external leakage and burning in the combustion chamber. Verify the condition by monitoring oil consumption and analyzing exhaust smoke.
Solution: This type of problem usually requires an engine disassembly to diagnose and repair.
Solution: Recondition the turbocharger (recore or replace).
Solution: Replace cylinder liners/sleeves or use a glaze-buster to machine crosshatch.
Solution: Measure to specification and recondition the cylinder head if required.
Verify the problem:
Some possible causes and suggested solutions:
Solution: Change the oil filter(s). If the problem persists, clean or replace the oil cooler bundle (core), and check or clean the filter and oil cooler bypass valves.
Engine lube contaminated with fuel can have darker appearance and feel thin to the touch. If the cause is fuel, locate the source. This may be difficult and the procedure varies with the type of fuel system and the routing of the fuel to the injector.
Solution: Pressure testing the fuel delivery components may be required. Porosity in the cylinder head casting, failed injector O-ring seals, leaking fuel jumper pipes, and cracked cylinder head galleries are some possible causes. Perform repairs as required, and then service the oil and filters.
Solution: Replace the bearings, ensuring that the clearance of the new bearings is checked.
Solution: Clean the valve and housing, replacing parts as necessary. Check the bypass/diverter valves in the oil cooler and filter mounting pad.
Solution: Recondition or replace the oil pump.
Solution: Replace the oil suction pipe.
Solution: Replace the oil pressure gauge or sending unit.
Solution: Change the oils and filters.
A variety of engine noises may occur. Although some noises have little significance, others can indicate serious engine trouble that will require prompt attention to prevent major damage to the engine. A listening rod can be of help in locating the source of a noise. The rod acts somewhat like the stethoscope a doctor uses to listen to a patient's heartbeat or breathing. When one end is placed at your ear and the other end at some particular part of the engine, noises from that part of the engine will be carried along the rod to your ear. By determining the approximate source of the noise, you can, for example, locate a broken or noisy ring in a particular cylinder or a main bearing knock.
When checking hydraulic valve lifters, remember that grit, sludge, varnish, or other foreign matter will seriously affect operation of these lifters. If you find any foreign substance in the lifters or engine where it may be circulated by the lubrication system, you need to do a thorough cleaning job to avoid a repetition of lifter trouble.
To help prevent lifter trouble, change the engine oil and oil filter as recommended in the service manual. Faulty valve lifter operation usually appears under one of the following conditions:
Rapping noise only when the engine is started. When engine is stopped, any lifter on a camshaft lobe is under pressure of the valve spring; therefore, leak down or escape of oil from the lower chamber can occur. When the engine is started, a few seconds may be required to fill the lifter, particularly in cold weather. If noise occurs only occasionally, it may be considered normal, requiring no correction. If noise occurs daily, however, check for (a) oil too heavy for prevailing temperatures, or (b) excessive varnish in lifter.
Intermittent Rapping Noise. An intermittent rapping noise that appears and disappears every few seconds indicates leakage at the check ball seat due to foreign particles, varnish, or defective surface of the check ball or seat. Recondition, clean, and/or replace lifters as necessary.
Noise at idle and low speed. If one or more valve lifters are noisy on idle at up to approximately 25 mph but quiet at higher speeds, it indicates excessive leak down or faulty check ball seat on the plunger. With engine idling, lifters with excessive leak down rate may be spotted by pressing down on each rocker arm above the push rod with equal pressure. Recondition or replace noisy lifters.
Generally noisy at all speeds. Check for high oil level in crankcase. With the engine idling, strike each rocker arm above the push rod several sharp blows with a mallet; if the noise disappears, it indicates that foreign material was keeping the check ball from seating. Stop the engine and place the lifters on the camshaft base circle. If there is lash clearance in any valve train, it indicates a stuck lifter plunger, worn lifter body lower end, or worn camshaft lobe.
Loud noise at normal operating temperature only. If a lifter develops a loud noise when the engine is at normal operating temperature, but is quiet when engine is below normal temperature, it indicates an excessively fast leak down rate or scored lifter plunger. Recondition or replace the lifter.
There are two distinct types of rod knock. The gudgeon pin end or wrist pin or little end bearing makes a light metallic clack. Often you can hear the double clack as the connecting rod reverses the piston direction. The main connecting rod bearing or big end makes a loud, deeper toned knock and is very bad news. Engines can run for a surprisingly long time with little end knock. Big end knock is usually rapidly terminal.
Disconnecting the spark plug wire (and carefully grounding it in the case of high energy ignition!) and then running the engine will cause almost all little end knocks to disappear. The main source of little end knock is the combustion event hammering the piston down onto the bearing, taking up the excess clearance with a clack. If the wrist pin is really loose, then disconnecting the spark plug can change a double clack to a single as the lightly loaded piston reverses direction.
Disconnecting the spark plug rarely makes big end knock disappear altogether.
The really troublesome thing about connecting rod problems is if it does cease, a new block will be required, so the risk of continuing to run the engine is quite high. Always run an engine at low rpm if you suspect rod knock of either type. Not generally known is the fact that inertia loads on the TDC reversal of the piston on the exhaust to intake stroke is when loads on the connecting rod peak due to no downward pressure on the piston face and high speed loading of the bearings.
The noise a connecting rod makes can be similar to detonation (the cause of the sound is similar, the cylinder rings with the banging of the piston on the rod bearings in the one case, and with the sound of the detonation wave hitting the piston face in the other), but a shot connecting rod should make noise at idle. Also, changing ignition timing or increasing octane will cause pinging to go away, but a worn connecting rod will continue to knock.
Piston-pin knock is a sharp, metallic rap that can sound more like a rattle if all the pins are loose. It originates in the upper portion of the engine and is most noticeable when the engine is idling and the engine is hot. Piston-pin knock sounds like a double knock at idle speeds. It is caused by a worn piston pin, piston pin boss, piston pin bushing, or lack of lubrication resulting in worn bearings. To correct the problem, install oversized pins, replace the boss or bushings, or replace the piston.
This sound can be heard during acceleration as high-pitched rattling or clicking in the upper part of the cylinder. It can be caused by worn rings or cylinders, broken piston ring lands, or insufficient ring tension against the cylinder walls. Ring noise is corrected by replacing the rings, pistons, or sleeves or reboring the cylinders
Piston slap is commonly heard when the engine is cold and often gets louder when the vehicle accelerates. When a piston slaps against the cylinder wall, the result is a hollow, bell-like sound. Piston slap is caused by worn pistons or cylinders, collapsed piston skirts, misaligned connecting rods, excessive piston-to-cylinder wall clearance, or lack of lubrication resulting in worn bearings. Correction requires either replacing the pistons, reboring the cylinder, replacing or realigning the rods, or replacing the bearings. Shorting out the spark plug of the affected cylinder might quiet the noise.
Crankshaft knock is a heavy, dull, metallic knock that is noticeable when the engine is under load or accelerating. When the noise is regular, it can be contributed to worn main bearings. When irregular and sharp, the noise is probably due to worn thrust bearings.
The Navy provides accurate and dependable testing equipment for use within the mechanic shops. But having the testing equipment in the shop is NOT enough. The supervisor and the crew must know how to use this equipment since proper use provides the quickest and surest means of finding out what is wrong and where the fault lies with an engine or component. Three of the most widely testing instruments used to check an engine for mechanical problems are the cylinder compression tester, vacuum gauge, and cylinder leakage tester.
The engine power results from igniting a combustible mixture that has been compressed in the combustion chamber of an engine cylinder. The tighter a given volume of fuel mixture is squeezed in the cylinder before it is ignited, the greater the power developed. Unless approximately the same power is developed in each cylinder, the engine will run unevenly. The cylinder compression tester is used to measure cylinder pressure in psi as the piston moves to TDC on the compression stroke (Figure 13).
Figure 13 - Compression tester kit.
Some terms associated with the compression are the following:
Figure 14 - Compression ratio.
In measuring compression pressures of all cylinders with a compression gauge, the different cylinders of an engine may vary as much as 20 pounds. The variation is caused largely by the lack of uniformity in the volume of the combustion chamber. It is nearly impossible to make all the combustion chambers in a cylinder head exactly the same size. For example, in a given engine with chambers the same volume, the compression pressure would be about 120 pounds in all cylinders. However, if one combustion chamber is 1/3 cubic inch too small, the pressure will be about 126 pounds, and if it is 1/3 cubic inch too large, the compression pressure would be about 114 pounds. This is a variation of 12 pounds. Also note that a carbon deposit will raise the compression pressure at any given ratio by reducing the combustion chamber volume-the greater the deposit, the higher the pressure.
To make a compression test, first warm up the engine. Warming up will allow all the engine parts to expand to normal operating condition and will ensure a film of oil on the cylinder walls. Remember that the oil film on the walls of the cylinder helps the expanded piston rings seal the compression within the cylinder. After the engine is warmed to operating temperature, shut it down and remove all the spark plugs. Removing all the plugs will make the engine easier to crank while you obtain compression readings at each cylinder. The throttle and choke should be in a wide-open position when compression readings are taken. Some compression gauges can be screwed into the spark plug hole. Most compression gauges, however, have a tapered rubber end plug and must be held securely in the spark plug opening until the highest reading of the gauge is reached.
Crank the engine with the starting motor until it makes at least four complete revolutions. Normal compression readings for gasoline engine cylinders are usually 100 psi or slightly higher. Compression testing is faster and safer when there are two mechanics assigned to the job. Remember that the compression test must be completed before the engine cools off.
Unless the compression readings are interpreted correctly, it is useless to make the tests. Any low readings indicate a leakage past the valves, piston rings, or cylinder head gaskets. Before taking any corrective action, make another check to try to pinpoint the trouble. Pour approximately a tablespoon of heavy oil into the cylinder through the spark plug hole, and then retest the compression pressure. If the pressure increases to a more normal reading, it means the loss of compression is due to leakage past the piston rings. If adding oil does not help compression pressure, the chances are that the leakage is past the valves. Low compression between two adjacent cylinders indicates a leaking or a blown head gasket. If the compression pressure of a cylinder is low for the first few piston strokes and then increases to near normal, a sticking valve is indicated. Near normal compression readings on all cylinders indicate that the engine cylinders and valves are in fair condition. Indications of valve troubles by compression tests may be confirmed by taking vacuum gauge readings.
When an engine has an abnormal compression reading, it is likely that the cylinder head will have to be removed to repair the trouble. Nevertheless, the mechanics should test the vacuum of the engine with a gauge. The vacuum gauge provides a means of testing intake manifold vacuum, cranking vacuum, fuel pump vacuum, and booster pump vacuum. The vacuum gauge does NOT replace other test equipment, but rather supplements it and diagnoses engine trouble more conclusively.
Vacuum gauge readings are taken with the engine running and must be accurate to be of any value. Therefore, the connection between the gauge and intake manifold must be leak proof. Also, before the connection is made, see that the openings to the gauge and intake manifold are free from dirt or other restrictions. When a test is made at an elevation of 1,000 feet or less, an engine in good condition, idling at a speed of about 550 rpm, should give a steady reading of from 15 to 22 inches on the vacuum gauge. The average reading will drop approximately 1 inch of vacuum per 1,000 feet at altitudes of 1,000 feet and higher above sea level.
When the throttle is opened and closed suddenly, the vacuum reading should first drop to about 2 inches with the throttle open, and then come back to a high of about 24 inches before settling back to a steady reading as the engine idles. This is normal for an engine in good operating condition.
If the gauge reading drops to about 15 inches and remains there, it would indicate compression leaks between the cylinder walls and the piston rings or power loss caused by incorrect ignition timing; a vacuum gauge pointer indicating a steady 10, for example, usually means that the valve timing of the engine is incorrect. Below normal readings that change slowly between two limits, such as 14 and 16 inches, could point to a number of troubles. Among them are improper carburetor idling adjustment, maladjusted or burned breaker points, and spark plugs with the electrodes set too closely.
A sticking valve could cause the gauge pointer to bounce from a normal steady reading to a lower reading and then back to normal. A broken or weak valve spring would cause the pointer to swing widely as the engine is accelerated. A loose intake manifold or a leaking gasket between the carburetor and manifold would show a steady low reading on the vacuum gauge.
Vacuum gauge tests only help to locate the trouble. They are not always conclusive, but as you gain experience in interpreting the readings, you can usually diagnose engine behavior.
Vacuum Gauge.Few tools or test devices are more useful and versatile than a vacuum gauge. A vacuum gauge can tell as much about the internal and external workings of an engine as the combination of a voltmeter, compression gauge, stethoscope, and timing light. An engine's vacuum readings can provide information about its running parameters, provided you know how to read the gauge.
Atmospheric pressure is measured at 14.7 pounds/square inch at sea level. That pressure corresponds to the weight of air holding a column of mercury 29.92 inches in height. By definition, vacuum is pressure below normal atmosphere, commonly caused by a suction that is taking the air molecules away from a particular location. In engines, of manual, the air is being sucked in by the vacuum created by the movement of the pistons.
All vacuum gauge faces are graduated in inches of mercury, although some have additional scales in millimeters of mercury.
Note also that most vacuum gauges are equipped with an additional scale that measures fuel pump pressure. This allows the user to connect the hose directly to the fuel line entering the carburetor to measure the fuel pump's pressure. Since many fuel system problems can be traced to the pumps themselves, such a tool can save a lot of troubleshooting time. Is this a great tool, or what?
The most important thing to do when using a vacuum gauge is to connect it to a constant vacuum source on the engine. Some manifolds incorporate a plug that may be removed for such purposes. If none exists, the next best place to connect is the PCV hose. If that's too hard to reach, connect to the power brake vacuum hose (on the engine side of the one-way valve in the hose).
Finally, you can connect to the vacuum line at the carburetor, but make sure the line has vacuum at idle. Many distributors were designed to get advance vacuum only when the throttle plate was opened, in which case there was no vacuum at idle. Make sure you've connected things properly.
Vacuum Gauge Scenarios. The following scenarios are typical examples and are not absolute "drop-dead" numbers. Read the maintenance manuals for the correct reading for your engine.
Normal Vacuum Reading. Normal vacuum reading is typically 15-22 inches, with needle holding steady. This means that the engine's compression is fine and there are no vacuum leaks or ignition anomalies.
- Rapid acceleration: When the engine is accelerated its vacuum will initially drop close to zero and then slowly start to rise. This is because the wide-open throttle plate momentarily allows atmospheric pressure in, equalizing the pressure in the intake system.
- Deceleration: If the engine is decelerated (throttle plate closed while engine is revved up) the vacuum will momentarily go way up, frequently to 25-30 inches, and then drop to normal. This is due to relative inequality between the closed (throttle plate) intake system and the cylinder's suction.
High Performance Engines.High performance engines-those with high-lift, long duration, large overlap-will show a normal vacuum reading lower than "stock" engines, typically around 15 inches. The needle will remain steady, but a little "needle shake" is to be expected. This is because of valve overlap, where both intake and exhaust valves are momentarily open together.
Engine with Worn Rings or Diluted Oil. Normal operation shows about 15-17 inches. Rapid acceleration causes the needle to rise only to about 20-23 inches (normal would be 26-30). Reason? Lower vacuum due to gases bypassing the rings.
Sticking Valves. If the problem is sticking valves, the needle remains steady, but quickly flicks down and then back up. The drop downward is usually around 3-4 inches on the scale. This flick of the needle occurs when a particular faulty valve is actuated.
Burned/Leaking Valves. Engines that have burned or constantly-leaking valves will exhibit vacuum readings. There is an evenly-spaced downward flicking of the needle, usually over about 6-8 inches.
Poorly Seated Valves. A regularly downward flicking of the needle over just 2-4 inches indicates poorly seated valves. This is in contrast to quick movements of the needle.
Worn Valve Guides. When you see the needle regularly swing back and forth over 4-6 inches, it means the valve guides are worn. You can check this diagnosis by gradually increasing the engine speed. If the needle grows steady, you can assume the guides are worn.
Weak Valve Springs. When the needle oscillates violently over 10-14 inches as the engine speed is increased, it means there are weak valve springs. In this condition the reading at idle can be relatively steady.
Valve Timing/Intake Leak. A low, steady reading of about 10 inches indicates late valve timing or a possible slight intake leak. To see if the problem is a leak, spray starter fluid over the intake area where it is bolted to the heads. If the engine smoothes up or increases speed, there is a leak. Late valve timing means lots of work to correct things.
Retarded Ignition Timing. A steady but "mediocre" needle reading of about 15 inches indicates retarded ignition timing. This is easy to fix rotation of the distributor. If the same reading is accompanied by a regular pulsation (not a flicker) of the needle, check the gap on the spark plugs or for defective breaker points.
Major Intake Leak. A low, steady reading of 3-6 inches means there is a major intake leak. Check the carburetor mounting flange gaskets and manifold gaskets. If nothing is found, go through each vacuum hose on the vehicle looking for loose connections or split hoses.
Blown Gasket. If the needle starts off at normal vacuum but drops off in a regular manner, suspect a blown gasket. Generally, such problems are accompanied by smoke and other more obvious symptoms, so very few head gasket failures are diagnosed by vacuum gauges.
Clogged Exhaust System. When an engine first starts and idles, the needle shows a normal reading, but as the engine speed increases the needle slowly goes to zero, suspect a clogged exhaust system. Excessive back pressure is the cause here.
Idle Mixture. If the carburetor's idle mixture is adjusted improperly, the vacuum gauge needle will move slowly back and forth between about 13-17 inches. Adjustment of the mixture screws will make the movement go away and return the needle to normal.
Another aid in locating compression leaks is the cylinder leakage test. The principle involved is that of simulating the compression that develops in the cylinder during operation. Compressed air is introduced into the cylinder through the spark plug or injector hole, and by listening and observing at certain key points you can make some basic deductions.
The cylinder leakage tester is used to test cylinder leakage by applying regulated air to the cylinder at a controlled volume and pressure and producing a percentage of leakage specification. There are commercial cylinder leakage testers, (Figure 15) available, but actually the test may be conducted with materials readily available in most repair shops. In addition to the supply of compressed air, a device for attaching the source of air to the cylinder is required. For a gasoline engine, this device can be made by using an old spark plug of the correct size for the engine to be tested. By removing the insulator and welding a pneumatic valve stem to the threaded section of the spark plug, you will have a device for introducing the compressed air into the cylinder.
The next step is to place the piston at TDC or "rock" position between the compression and power strokes. Then you can introduce the compressed air into the cylinder. Note that the engine will tend to spin. Now, by listening at the carburetor, throttle body or intake manifold, the exhaust pipe, and the oil filler pipe (crankcase), and by observing the coolant in the radiator, when applicable, you can pinpoint the area of air loss. A loud hissing of air at the intake manifold area would indicate a leaking intake valve or valves. Excessive hissing of air at the oil filler tube (crankcase) would indicate an excessive air leak past the piston rings. Bubbles observed in the coolant at the radiator would indicate a leaking head gasket.
Figure 15 - Cylinder leakage tester
As in vacuum testing, indications are not conclusive. For instance, the leaking head gasket may prove to be a cracked head, or the bad rings may be a scored cylinder wall. The important thing is that the source of trouble has been pinpointed to a specific area, and a fairly broad, accurate estimate of the repairs or adjustments required can be made without dismantling the engine.
In making a cylinder leakage test, remove all the spark plugs so that each piston can be positioned without the resistance of compression of the remaining cylinders. The commercial testers, such as the one shown in Figure 15, have a gauge indicating a percentage of air loss. The gauge is connected to a spring-loaded diaphragm. The source of air is connected to the instrument and counterbalances the action of the spring against the diaphragm. By adjusting the spring tension, you can calibrate the gauge properly against a variety of air pressure sources within a given tolerance.
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As a supervisor, you will be responsible for the care and maintenance of the engine testing equipment, such as the cylinder compression tester, vacuum gauge, and cylinder leakage tester. As the supervisor, you must impress upon the less experienced mechanics that these precision gauges and testers are fragile instruments that can be damaged through improper use or rough handling. They should be put back in the case they came in and returned to a safe place in the tool room immediately after being used so they are not lying around with the possibility to be damaged. Keeping the gauges and testers clean is about all the maintenance that is required if handled properly. If they are dropped, stepped on, broken, or jarred out of calibration, the gauges or testers will probably have to be returned to the manufacturer for repair or replacement.
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Even if an engine has been properly maintained and serviced, the first major repair the engine will need is an overhaul involving the valves.
Some of the common valve troubles that you may encounter in working with engines, and their possible causes are indicated below.
Sticking valves will produce unusual noise at the cam followers, pushrods, and rocker arms and may cause the engine to misfire. Sticking is usually caused by resinous deposits left by improper lube oil or fuel. The cause could also be due to worn valve guides, a warped valve stem, insufficient oil, cold engine operation, or overheating.
To free sticking valves without having to disassemble the engine, use one of several approved commercial solvents. If the engine is disassembled, use either a commercial solvent or a mixture of half lube oil and half kerosene to remove the resins.
Do not use the kerosene mixture on an assembled engine since a small amount of this mixture settling in a cylinder could cause a serious explosion.
Bent valves or slightly warped valves tend to hang open. A valve that hangs open not only prevents the cylinder from firing, but also is likely to be struck by the piston and bent so that it cannot seat properly. Bent valves are generally from the meeting between the piston and valve. Causes here typically include a broken timing chain or belt, weak or broken valve springs, over revving the engine, valve sticking (insufficient guide clearance or lubrication, overheating, etc.) and insufficient valve-to-piston clearance (excessive valve lift, valve reliefs not cut deeply enough into pistons, wrong pistons, not enough deck height, heads milled too much, etc.). Symptoms of warped or slightly bent valves will usually show up as damage to the surface of the valve head. To lessen the possibility that cylinder head valves will be bent or damaged during overhaul, NEVER place a cylinder head directly on a steel deck or grating; use a protective material such as wood or cardboard. Also, NEVER pry a valve open with a screwdriver or similar tool.
Burned valves are indicated by irregular exhaust gas temperatures and sometimes by excessive noise. In general, the primary causes of burned valves are a sticking valve, insufficient valve tappet clearance, a distorted seat, overheated engine, lean fuel-air mixture, pre-ignition, detonation, valve seat leakage or valve heads that have been excessively reground.
The principal cause of burned exhaust valves is small particles of carbon that lodge between the valve head and the valve seat. These particles come from incomplete combustion of the fuel or oil left by the piston rings in the cylinder. The particles hold the valve open just enough to prevent the valve head from touching the valve seat. The valve is cooled by several means, including its contact with the valve seat. When carbon particles prevent contact, the heat normally transferred from the valve head to the seat remains in the valve head. The valve seat seldom burns because the water jackets surrounding the seat usually provide enough cooling to keep its temperature below a dangerous point.
Valve breakage can happen to either the intake or exhaust valves, and they generally break in one of two places, where the head is joined to the stem, or where the keeper grove(s) are machined into the end of the stem. Nevertheless, breakage of the valves is destructive due to the pieces falling into the combustion chamber and inflicting damage on the piston and head. Valve breakage may occur by valve overheating, detonation, excessive tappet clearance, seat eccentric to stem, cocked spring or retainer, or scratches on the stem caused by improper cleaning.
Loose valve seats can be installed only by installing them properly. Clean the counter bore thoroughly to remove all carbon before shrinking in an insert. Chill the valve seat with dry ice and place the cylinder head in boiling water for approximately 30 minutes; then drive the insert into the counter bore with a valve insert installing tool. Never strike a valve seat directly. Conduct the driving-in operation quickly, before the insert reaches the temperature of the cylinder head. When replacing a damaged valve with a new one, inspect the valve guides for excessive wear. If the valve moves from side to side as it seats, replace the guides.
When mechanical solid lifters are used, the valves require periodic adjustment. Proper and uniform valve adjustments are required for a smooth running engine. Incorrect valve adjustment will upset the amount of air-fuel mixture pulled into the cylinder. It also affects valve lift and duration. This will affect combustion and reduce engine efficiency.
Unless the clearance between valve stems and rocker arms or valve lifters is adjusted according to the manufacturer's specifications, the valves will not open or close at the proper time, and engine performance will be affected. Too great a clearance will cause the valves to open late. Excessive clearance may also prevent a valve from opening far enough and long enough to admit a full charge of air or fuel mixture (with either a diesel or gasoline engine), or it will prevent the escape of some exhaust gases from the cylinder. A reduced charge in the cylinder obviously results in engine power loss. Exhaust gases that remain in the cylinder take up space, and when combined with the incoming charge, reduce the effectiveness of the mixture. Valves adjusted with too little clearance will overheat and warp. Warped valves cannot seat properly and will permit the escaping combustion flame to burn both the valve and valve seat.
When reassembling an engine after reconditioning the valves, make sure the adjusting screws are backed off before rotating the engine. A valve that is too tight could strike the piston and damage either the piston or the valve, or both. Adjust the valves according to the manufacturer's specifications, following the recommended procedure.
On any engine where valve adjustments have been made, be sure that the adjustment locks are tight and that the valve mechanism covers and gaskets are in place and securely fastened to prevent oil leaks.
There are several different methods of adjusting the valves on an overhead cam engine. In many overhead cam designs, the valves are adjusted like the mechanical lifters in a push rod engine. Most overhead valves are adjusted "hot", that is, valve clearance recommendations are given for an engine at operating temperatures. Before valve adjustments can be properly effected, the engine must be run and brought up to normal operating temperature.
To adjust a valve, remove the valve cover and measure the clearance between the valve stem and the rocker arm. Loosen the locknut and turn the adjusting screw in the rocker arm, in the manner shown in Figure 16. On engines with stud-mounted rocker arms, make the adjustment by turning the stud nut. A rocker arm adjustment screw is turned until the correct size feeler gauge fits between the cam lobe and the follower, valve shim, or valve stem.
Figure 16 Adjusting the valves on an overhead cam engine.
To remove the valves from the cylinder head, the valve spring retainers and the valve springs must be removed. To aid in spring and retainer removal, use a valve spring compressor to compress the spring (Figure 17). First adjust the tool to fit the valve assembly and then proceed as follows:
Valve springs are under considerable tension and can fly from the cylinder head with considerable force. Wear proper face and eye protection when compressing springs and removing valve keepers.
Figure 17 -Valve spring compressor.
Valve grinding is done by machining a fresh, smooth surface on the valve faces and the valve stem tips. Valve faces can burn, pit, and wear as the valves open and close during engine operation. Valve stem tips wear because of friction from the rocker arms or followers.
Wear a face shield when grinding valves. The grinding stone could shatter, throwing debris into your face.
A valve grind machine is used to resurface valve faces and stems (Figure 18). Although there are some variations in design, most valve grind machines are similar. They use a grinding stone to remove a thin layer of metal from the valve face and the valve stem tip.
Figure 18 - Valve grind machine. 4-33
Before grinding a valve, dress the stone by using a diamond-tipped cutting attachment to true the grinding surface. The cutting tool is generally provided with the valve grind machine. Follow the equipment manufacturer's instructions.
Be careful when using a diamond cutting tool to dress a stone. Wear eye protection and feed the tool into the stone slowly. If the tool is fed in too fast, tool or stone breakage may occur.
The chuck angle is generally set by loosening a lockout and rotating the grinding machine's chuck assembly to the desired cutting angle. A degree scale is provided so that the angle can be precisely adjusted.
An interference angle (a 1° difference between valve face angle and valve seat angle) is recommended for some older engines. If, for example, the valve seat angle is 45°, the chuck is set to grind the valve face to 44°. The interference angle provides a thin line of contact between the valve face and the valve seat, reducing the valve's break-in time. Older engines, which have hardened valves and seats, do not require an interference angle. The valve face and valve seat should be ground to matching angles.
Direct the coolant on the valve head while grinding. Move the valve into the stone slowly to prevent valve or stone damage. Only cut or grind a valve face enough to remove the dark pits. If the valve face is shiny all the way around, stop grinding.
Chuck the valve in the valve grind machine by inserting the valve stem into the chuck. Make sure the stem is inserted so that the chuck grasps the machined surface nearest the valve head. The chuck must not clamp onto an unmachined surface or runout will result.
Turn on the valve grind machine and the cooling fluid. Gradually feed the valve face into the stone. At the same time, slowly move the valve back and forth over the stone. Use the full face of the stone, but do not let the valve face move out of contact with the stone while grinding.
Grind the valve only long enough to "clean up" its face. When the full face looks shiny, with no darkened pits, shut off the machine and inspect the face. Look carefully for pits and grooves.
Some valve refacing machines are equipped with a carbide tip instead of a grinding wheel.
Grinding, by removing metal from the valve face, will increase valve stem height (distance the valve stem extends above the surface of the head). This affects the spring tension and valve train geometry. Grind the face of each valve as little as possible.
If the valve head wobbles as it turns on the valve grind machine, the valve is either bent or chucked improperly. Shut off the machine and find the cause.
A sharp valve margin indicates excess valve face removal and requires valve replacement. Manufacturers give a specification for minimum valve margin thickness. If the margin is too thin, it will not dissipate heat fast enough. The head of the valve can actually begin to melt, burn, and blow out of the exhaust port.
If not noticed during initial inspection, a burned valve will show up during grinding operations. Excess grinding will be needed to remove the deep pits and grooves found on a burned valve. Replace the valve if it is burned.
Repeat the grinding and inspecting operation on the other valves. Return each ground valve to its place in an organizing tray. Used valves should be returned to the same valve guide in the cylinder head. The stems may have been select-fit at the factory.
A second stone on the valve grind machine is normally provided for truing the valve stem tips.
After grinding the tip flat, you may have to chamfer the tip. If so, mount the valve in the V-block chuck that is at an angle to the stone. Slowly rotate the valve in the chuck as you feed the tip into the stone.
Grind as little off the stem tip as possible. Many stems are hardened. Too much grinding will cut through the hardened layer and result in rapid wear when the valve is returned to service.
An indicator is provided on the valve grind machine to show the depth of cut for both the valve face and the valve stem tip. Generally, the same amount of metal should be removed from the face and the tip. This will help keep valve train geometry correct.
Valve guides support the valves in the cylinder head. They are machined to a diameter that is a few thousandths of an inch larger than the diameter of the valve stems, providing a very small clearance between the stem and the guide. This clearance is important for a number of reasons. It prevents the lubricating oil from being drawn past the valve stem and into the combustion chamber during the intake stroke, prevents burnt gases from entering the crankcase area past the valve stems during the exhaust stroke, and helps keep the valve cool. The small valve guide clearance also keeps the valve face in perfect alignment with the valve seat.
Although valve guides may be cast with the head, removable valve guide inserts are used in most cases. Removable guide inserts are press-fit into the cylinder head, (Figure 19).
Figure 19 - Valve guides.
Before checking valve stem-to-guide clearance, clean the valve stem with solvent to remove all gum and varnish. Clean the valve guides with solvent and/or a wire-type expanding valve guide cleaning tool. Then insert the proper valve into its guide and hold the head against the valve seat tightly. Mount a dial indicator on the valve spring of the cylinder head so that the indicator's foot rests against the valve stem at a 90° angle.
Move the valve slightly off its seat and measure the valve guide-to-stem clearance by moving the stem back and forth to actuate the dial indicator. If the dial indicator reading is not within the manufacturer's specifications, determine if the stem or the guide is responsible for the excessive clearance.
In an alternate method of checking the valve stem-to-guide clearance, the dial indicator is mounted on the combustion side of the cylinder head. Before mounting the indicator, move the valve head away from its seat a predetermined distance, either by a special collar tool, which is placed on the valve stem between the head of the valve and the guide, or by simply measuring the distance between the valve head and the seat with a scale. After positioning the valve and mounting the dial indicator, move the valve head back and forth to actuate the indicator. If the indicator shows more clearance than specified, measure the valve stem with a micrometer. Compare the stem's diameter to specifications to determine whether the problem is caused by a worn valve guide or worn valve stem.
Other measuring techniques are available to the mechanic. A bore gauge, a micrometer, or an inside caliper-type small hole gauge can be used to determine valve guide wear. Remember, guides do not wear uniformly. Therefore, plugs, valve stems, or pilots should never be used to measure valve guides. Measure the inside diameter of the guides at several different points. Careful measurement and inspection of the guides will help detect bell mouthing or elliptical wear. These conditions normally occur at the ends of the guides.
Knurling is one of the fastest methods of restoring the inside diameter of a worn valve guide. It is the method usually chosen to recondition integral guides. Knurling raises the surface of the guide's inside diameter by cutting tiny furrows through the metal. As the knurling tool cuts into the guide, metal is raised or pushed up on either side of the cuts. This decreases the diameter of the guide hole. A burnisher or reamer is used to press the ridges flat and shave off the peaks. The final result produces the proper size hole and restores the correct guide-to-stem clearance. Knurling can be done with either a tap-type knurling tool or a power-driven tool. With power knurling, a drill speed reducer is usually required.
Reaming increases the diameter of the guide hole so it can be fitted with an oversized valve (valve with a larger stem). Reaming can also be used to restore the guide to its original diameter after installing inserts or knurling. The advantage of reaming for an oversized valve is that the finished product is totally new. This guide is straight, the valve is new, and the clearance is accurate. Installing an oversized valve is generally considered to be superior to knurling. The process is also relatively quick and easy. The only tool required is a reamer.
The use of thin-wall-inserts offers a number of important advantages over knurling and reaming. The liners provide the benefits of a bronze guide surface, including better lubrication, which reduces wear, and tighter clearances. Thin-wall liners can be used as either integral or replacement guides which are either cast iron or bronze. Thin-wall liners are faster, easier, and less expensive to install than new guides. Installing thin- wall guide liners also maintains guide centering with respect to the valve seats.
The use of threaded bronze inserts is another method used to restore worn guides. In this method, the worn guides are tapped and the threaded bronze inserts are installed in the tapped holes. They provide better lubrication, excellent wear qualities, and tight clearance, but they are more expensive and difficult to install.
After installing new valve seats, or when the old seats are in serviceable condition, grind or cut the faces of the seats. The equipment used to grind valve seats is a valve seat grinder (Figure 20).
Figure 20 - Valve seat grinding.
Two general types of valve seat grinders are in use. One is a concentric grinder, the other, an eccentric grinder. Only the concentric grinder is discussed here because of its greater availability.
In the concentric valve seat grinder, a grinding stone of the proper shape and angle is rotated in the valve seat, and the stone is kept concentric with the valve guide by means of a self-centering pilot that is installed in the guide. Check the self-centering pilot for trueness before using.
To grind a valve seat, install the correct size pilot (metal shaft that fits into the valve guide and supports the cutting stone or the carbide cutter). The pilot should fit snugly in the valve guide and should not wiggle. A damaged pilot will cause the seat position to move in relation to the valve guide. The valve guide must be kept clean and in good condition. Most concentric grinders automatically lift the stone off the valve seat about once every revolution to allow the stone to clean itself of dust and grit by centrifugal action.
Select the correct stone for the valve seat. It must be slightly larger in diameter than the seat and must also have the correct face angle.
Dress the stone using the diamond cutter provided with the grinding equipment. Set the cutter to the correct angle, usually 45° or 30°. Slowly feed the diamond cutter into the stone while spinning the stone with the power head. Cut only enough to clean up and true the stone.
To use hand cutters, follow the same procedures explained for grinding stones. Fit the correct size pilot securely into the guide. Select the correct diameter and angle cutter. Fit the cutter down over the pilot. Then, while applying a very light downward force, turn the cutter on the seat. Make sure you turn the cutter in the direction indicated by the arrow on the tool. Remove only enough material to clean up the seat and make it totally burnished.
To test the contact between the valve seat and the valve, mark lines with a soft pencil about 1/4 inch apart around the entire face of the valve. Next, put the valve in place and rotate, using a slight pressure, one-half turn to the right and then one-half turn to the left. If rotating removes the pencil marks, the seating is good.
Another method for checking the valve seating is to coat the valve face lightly with Prussian blue and turn it about one-fourth turn in the seat. If the Prussian blue transfers evenly to the valve seat, it is concentric with the valve guide. Be sure to wash all the Prussian blue from the seat and valve. Then lightly coat the valve seat with Prussian blue. If the blue again transfers evenly, this time to the valve when it is turned in the seat, you can consider the seating to be normal.
Prussian blue is a deep blue pigment mixed with a grease-like substance to stain metal that is often used to check the contact point between the valve face and valve seats. Apply a small amount of Prussian blue on the valve seat or face. Tap the valve down on the seat to mark the Prussian blue and make the contact point visible.
Valve seat replacement is needed when a valve seat is cracked, burned, pitted, or recessed in the cylinder head. Replacement is only needed when wear or damage is severe. Normally, valve seats can be machined and returned to service.
To remove a pressed-in valve seat, split the old seat with a sharp chisel. Then pry the seat out of the cylinder head. To remove an integral seat, use a seat-cutting tool to machine the seat from the cylinder head. Extreme care must be taken not to damage the head.
To install a valve seat, a technique used is to shrink the seat by chilling it in dry ice. The seat will expand when returned to room temperature. This helps lock the seat in the cylinder head. Use a driving tool to force the seat into the recess in the head. Seat installation tools vary. Follow the manufacturer's directions.
Stalking the valve seat involves placing small dents in the cylinder head next to the seat. The stakes force the head metal over the seat and keep the seat from falling out. Top cutting may be needed to machine the top of the seat flush with the surface of the combustion chamber.
Corrosion and metal fatigue are common causes of valve spring failure. After prolonged use, valve springs tend to weaken, lose tension, or even break. Broken valve springs cause excessive valve noise and may cause erratic exhaust gas temperatures. When a valve spring breaks, serious damage can occur. When a spring breaks, it may collapse enough to allow the valve to drop into the cylinder, where it can cause damage. Similarly, the valve stem keepers may release the valve and allow it to drop into the cylinder, causing severe damage to the piston, cylinder head, cylinder, and adjoining parts. During engine service, always test each valve spring to make sure it is in working condition.
Valve spring squareness, as shown in Figure 21, can be checked with a combination square. Place the spring next to the square on a flat surface. Rotate the spring while checking for a gap between the side of the spring and the square. Replace the spring if it is not square. Not more than 1/16" variance is the norm while rotating the spring.
Figure 21 - A combination square used to check valve spring squareness.
Valve spring free height can be measured with the combination square or with a valve spring tester (Figures 4-22). Simply measure the length of each spring in a normal, uncompressed condition. If too long or too short, replace the spring.
Figure 22 - A valve spring tester
Valve spring tension, or pressure, is measured with a valve spring tester (Figure 22). Compress the spring to the specified height and read the scale. Spring pressure must be within specifications. If spring pressure is too low, the spring has weakened and needs replacement or shimming. The valve spring tension must be within 10% of the original tension specification. If not, replace the valve.
There are two types of valve lifters: the solid type and the hydraulic type. Procedures for removing and servicing the two types are quite different.
Solid lifters are removed from the camshaft side on some engines. This requires removal of the camshaft. The lifters must be held up by clips or wires so that the camshaft can be extracted. Then the clips or wires are removed so that the lifters may be extracted. Most valve lifters can be extracted from the pushrod or valve side of the engine block, in which case extraction of the camshaft is not necessary. Be sure to keep the lifters in the proper order so that they may be replaced in the same bores from which they were removed.
If the lifter screw face is worn or pitted, it may be refaced on a valve re-facing machine. If the lifter bore in the block becomes worn, it maybe re-bored by reaming; then oversized lifters must be installed.
The contact surface between a lifter and a cam lobe is one of the highest friction and wear points in an engine. Hydraulic lifters can also wear internally, causing valve clatter or tapping noise.
Inspect the bottom surface that contacts with the cam lobe of each lifter for wear. An unworn lifter will have a slight hump, or convex shape, on the bottom. A worn lifter will be flat or concave on the bottom. Replace the lifters if the bottom is worn.
Never install used lifters on a new camshaft. Used lifters will cause rapid cam lobe wear and additional lifter wear. Install new lifters whenever a camshaft is replaced.
Lifter leak-down rate is measured by timing how long it takes to push the lifter plunger to the bottom of its stroke under controlled conditions.
On some engines, hydraulic lifters, shown in Figure 23, are tested by the leak-down- rate test. In testing, insert a feeler gauge between the rocker arm and the valve stem, and note the time it takes the valve lifter to leak enough oil to permit the valve to seat. As the valve seats, the feeler gauge becomes loose and signals the end of the test. If the leak-down-rate time is too short, the lifter is defective and must be replaced. In any case, be sure to follow the manufacturer's recommended procedures for performing this test.
Figure 23 - Hydraulic lifters.
To remove the hydraulic lifters, remove the pushrod. On engines with shaft-mounted rocker arms, you can move the rocker arm by compressing the spring so that the pushrod can be removed. Thus, the rocker arm assembly does NOT have to be removed.
After the lifter has been removed, check the bottom or cam side to ensure that it is flat. To do this, place a straightedge across the lifter bottom. If light can be seen between the straightedge and the lifter, the lifter should be discarded.
When disassembling the lifter, be sure to clean all the parts in a cleaning solvent. Reassemble and fill the lifter with clean, light engine oil. Also, make sure that all lifters are replaced in the same bore from which they were removed. Work on one lifter at a time so that parts are not mixed
The camshaft must be checked for bearing journal or cam wear and alignment. In checking alignment, place the camshaft in a set of V-blocks, and use a dial indicator to check the runout of the journals when the shaft is turned. Journals should be checked with a micrometer and the reading compared to the manufacturer's specifications. The cam wear should be measured with a micrometer; however, if wear shows across the full face of the cam, you can be almost certain that excessive wear has taken place.
Cam bearing diameter indicates cam bearing wear. Measure bearing diameter with a bore gauge or a telescoping gauge and an outside micrometer (Figure 24). A-Dial bore gauge is used to quickly and accurately measure cylinder wear. B-Telescoping gauge used to measure internal part bores or openings. C-Micrometer used for measuring external dimensions, diameters, or thicknesses.
If the bearings are worn, they must be replaced. Generally the mechanic will replace the cam bearings during an engine overhaul since they are critical to engine oil pressure.
Figure 24 - Tools used for camshaft bearing replacement
The two-piece cam bearings used on many OHC engines simply snap into place, like rod and main bearings. However, one-piece cam bearings must be forced in and out of the block or head with a special tool.
When installing cam bearings, do not dent or mar the bearing surfaces. Also, make sure you align the oil holes in the engine with the holes in the cam bearings. Since exact procedures vary, refer to the OEM for details.
The relationship between the camshaft and the crankshaft determines the valve timing. Gears, drive chains, and reinforced neoprene belts are used to drive the camshafts that open and allow the valves to close in relation to the position of the pistons in the cylinders. The gear drive sprockets, or cogs as the case may be, of the camshaft, and crankshafts are keyed in position so they cannot slip.
With directly driven timing gears, one gear usually has a mark on two adjacent teeth, and the other, a mark on only one tooth. To time the valves properly, you need to mesh the gears so that the two marked teeth of the one gear straddle the single marked tooth of the other gear. In chain-driven sprockets, you can obtain correct timing by having a certain number of chain teeth between the marks or by lining up the marks with a straightedge. Engines using a continuous neoprene belt have sprockets, or cogs, attached to the camshaft and crankshaft. The belt has square-shaped internal teeth that mesh with the teeth on the sprockets. All engines with this system use a timing belt tensioner. Timing marks on this system vary with each manufacturer.
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When removing bearing caps, if they are not already marked, be sure to mark them so they will be replaced on the same journal from which they were removed. If bearing caps stick, carefully work them loose by using a soft-faced hammer, to avoid distorting them, and tapping the cap lightly on one side and then the other.
The crankshaft is one of the most highly stressed engine components. The stress increases four times as the engine speed doubles. The crankshaft must be rejected if there is any sign of a crack because a cracked crankshaft may break if it continues in service (Figure 25). Crankshaft cracks in can be detected initially with a close visual inspection.
Figure 25 - A broken crankshaft.
Once the bearing caps have been removed, lift the crankshaft out of the engine block. Usually one or two people do this seemingly simple operation by hand. With larger crankshafts, use a hoist as demonstrated in Figure 26 by lifting above the center with a rope sling around two of the throws.
Figure 26 - Removing the crankshaft with a hoist.
Do not "bang" around or put unnecessary stress on the crankshaft, causing damage that will have to be repaired prior to the crankshaft being re-installed into the engine, which will result in needless down time and expense.
Journals wear out-of-round and become tapered. Out-of-round and taper are measured using a micrometer to take measurements at a number of different locations on each journal. Rough journals and slight bends can be rectified by grinding the journals on true centers. Forged shafts with excess bend should be straightened before grinding.
The preferred method of measuring crankshaft journals is as follows. Remove the crankshaft from the engine block and clean the surfaces to be measured. Using the outside micrometer, measure the journals at several points from side to side and from top to bottom across the bearing surface (Figure 27). Measurements around the journal will show if the journal is out of round.
Figure 27 - Measuring journals
Those measurements across the surface show if the journal is tapered. If one side of a crankshaft journal is worn more than the other, the journal is tapered. To measure journal taper, use the outside micrometer. Measure both ends of each journal. Journals that are tapered or out-of- round more than .003 must be reground. Be sure that you always refer to manufacturer's specifications when performing any crankshaft work. If not within spec limits, send the crankshaft for turning.
You should always check bearing fit or oil clearance when installing new bearings. When the bearing caps are off, you should measure the journals so that you can detect wear, out of roundness, or taper. You can check bearing clearance with either feeler stock or Plastigage (Figure 28).
Figure 28 - Using Plastigage to check bearing clearance.
Plastigage is a small plastic string that is used to measure a very tight clearance between two engine half circle metal bearings that enclose a spinning circular crankshaft, camshaft, or piston tie rod end.
Plastigage is cut and laid across the width of the contact surface. The two half circle bearings are assembled and tightened to factory specifications, then disassembled to examine the Plastigage width.
The width of the compressed Plastigage indicates clearance between the parts. The range of Plastigage is determined by its color: green for 0.001-0.003-inch, red for 0.002- 0.006, and blue for 0.004-0.009-inch clearances.
Before checking bearing clearance with Plastigage, wipe the journal and the bearing clean of oil. Then place a strip of the Plastigage lengthwise in the center of the bearing cap. Install the cap next and tighten it into place. When the cap is removed, you can measure the amount of flattening of the strip with a special scale. Do NOT remove the flattened strip from the cap or the journal to measure the width, but measure it in place. Not only does the amount of flattening measure bearing clearance, but uneven flattening also indicates a tapered or worn crankshaft journal or bearing.
Do not turn the crankshaft with the Plastigage in place.
When using feeler stock to check main bearing clearances, place a piece of stock of the correct size and thickness in the bearing cap after it is removed. Coat the feeler stock lightly with oil. Then replace and tighten the bearing cap. Note the ease with which the crankshaft can be turned. As a word of caution, do not completely rotate the engine, which could damage the bearing. Turn it only about an inch in one direction or the other.
If the crankshaft is locked or drags noticeably after the bearing cap has been replaced and tightened, then the bearing clearance is less than the thickness of the feeler stock. If it does not tighten or drag, place an additional thickness of feeler stock on top of the first and again check the ease of crankshaft movement. Clearance normally should be about .002 inch. Be sure to check the engine manual for exact specifications.
Install the main bearing upper halves, following the order established during disassembly. Align the holes in each bearing shell with its mating port in the cylinder block. Apply a film of engine oil to each bearing wear surface and crankshaft journal. Lubricate the back of the bearings.
Make certain the crankshaft is clean prior to installing it in the cylinder block. Carefully lift and install the crankshaft into the cylinder block using a hoist and slings positioned to balance the weight. Slide the thrust washer upper halves into the cylinder block at the proper locations. Ensure the lubrication grooves in each bearing half are positioned to face toward the crankshaft wear surface. Install the main bearing lower halves onto their respective bearing caps, observing the order established during disassembly. At this point in the reassembly, a bearing clearance check using Plastigage may be performed.
Apply a film of engine oil to the wear surface of each bearing shell. Install the main bearing caps according to the order observed during disassembly. Install the bearing cap mounting bolts and washers. Snug the bolts but do not tighten, as the manual may specify a specific tightening sequence. After checking the service manual, tighten the bearing cap bolts to specification.
Crankshaft end play is the amount of front-to-rear movement of the crankshaft in the block. It is controlled by the clearance between the main thrust bearing and the crankshaft thrust surface or journal Crankshaft end play will become excessive if the thrust bearings are worn, producing a sharp, irregular knock. If the wear is considerable, the knock will occur each time the clutch is engaged or released; this action causes sudden endwise movement of the crankshaft.
Crankshaft end play should only be a few thousandths of an inch. To measure this end play, you can use a feeler gauge as shown in Figure 29, or mount a dial indicator on the block. Position the dial indicator against the crankshaft so that the indicator stem is parallel to the crank centerline.
Figure 29 - Checking crankshaft end play using a feeler gauge.
Pry the crankshaft back and forth in the block. Indicator movement equals crankshaft end play. Compare the measurements to the specifications. If the end play is incorrect, check the thrust bearing insert size and the crankshaft thrust journal width.
When removing the crankshaft with rigging, be sure to protect machined surfaces. Avoid laying down a crankshaft because it can warp out of shape or become bent. When storing a crankshaft, make certain it has support along the shaft's entire length to prevent warping. If the crankshaft is small enough, it may be possible to simply stand it on the flywheel end. Also, do not store crankshaft or bearing parts on any metal surface. When a shaft is removed from the engine, it should be placed on a wooden plank with all journal surfaces protected. If the shaft is to be exposed for some time, it is best to protect each journal surface with a coating of heavy grease.
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There are certain limits to which cylinders may become tapered or out-of-round before they require refinishing. If they have only a slight taper or are only slightly out-of-round (consult the manufacturer's manual for the maximum allowable taper or out-of-round), new standard rings can be installed.
When cylinder wear goes beyond the point recommended in the engine manufacturer's specifications, loss of compression, high oil consumption, poor performance, and heavy carbon accumulations in the cylinder will result. In such cases, the only way to put the engine back into good operating condition is to refinish the cylinders and fit new pistons (or oversized pistons) and rings.
As a first step in checking cylinder walls, wipe them clean and examine them carefully for scored places and spotty wear (which shows up as dark, unpolished spots on the walls). Holding a droplight at the opposite end of the cylinder from the eye will help in the examination. If scores or spots are found, you should refinish the cylinder walls.
Next, measure the cylinders for taper and oval wear. This can be done with an inside micrometer or by a special dial indicator. As the dial indicator is moved up and down in the cylinder and turned from one position to another, any irregularities will cause the needle to move. This will indicate how many thousandths of an inch the cylinder is out- of-round or tapered. The permissible amount of taper or out-of-roundness in a cylinder varies somewhat with different engines. Engine manufacturers issue recommendations based on experience with their own engines. When the recommendations are exceeded, the cylinders have to be refinished.
Cylinder taper is the difference in the diameters measured at the top of the cylinder and at the bottom of the cylinder. It is caused by less lubricating oil at the top of the cylinder and more oil splashing on the lower area of the cylinder. As a result, the top of the cylinder wears faster (larger) than the bottom, producing taper.
Cylinder out-of-roundness is a difference in cylinder diameter when measured front-to- rear and side-to-side in the block. Piston thrust action normally makes the cylinder wear more at right angles to the centerline of the crankshaft. Maximum allowable cylinder out-of-round is typically 0.00005".
A dial bore gauge is a tool used to quickly and accurately measure cylinder wear (Figure 30). Slide the bore gauge up and down the cylinder.
Figure 30 - Dial bore gauge.
There are two methods of refinishing cylinders: honing and boring. Cylinders are refinished by honing when wear is not too great; otherwise, they are bored with a machine, and oversized pistons and rings are installed.
Cylinder honing is used to true worn cylinders and to break the glaze on used cylinders before installing new piston rings. It must also be used to smooth rough cylinders after boring.
The term deglazing is generally used when referring to very light honing that simply scuffs the cylinder wall to aid ring break-in. The glazed cylinder wall causes rings to "skate" on the highly polished finish and discourages the minute amount of wear which is necessary to mate piston rings with the bore.
A cylinder hone produces precisely textured, cross-hatched pattern on the cylinder wall to aid ring seating and sealing. Tiny scratches from the hone cause initial ring and cylinder wall break-in wear. This makes the ring fit in the cylinder perfectly after only a few minutes of engine operation.
There are several types of hones:
Figure 31 - Type of hones.
A rigid hone or a honing machine can be used like a boring bar to true a cylinder when wear does not exceed acceptable limits. Do not hone more than manufacturer's recommendations.
After honing, it is very important to remove all honing grit (bits of stone and metal) from inside the engine. If this grit is not removed, it will act like grinding compound on bearings, rings, and other vital engine parts.
First, wash out the cylinders with a warm solution of water and soap. A soft bristle brush, not a wire brush, will quickly loosen particles inside the honing marks on the cylinder walls. After washing, rinse the block thoroughly with clean, hot water and blow the cylinders dry with compressed air.
Next, soak a clean shop rag in fresh engine oil and wipe the cylinder down thoroughly with the oil-soaked rag. The heavy oil will pick up any remaining grit embedded in the cylinder's honing marks. Wipe the cylinders down until the rag comes out perfectly clean.
After cleaning and oiling, recheck the cylinder for scoring and scratches. If honing did not clean up all the vertical scratches in the cylinder, cylinder boring or sleeving may be needed.
Cylinder boring is needed to remove deep scratches, scoring, or excess wear from the cylinder walls. It involves machining the cylinders to a larger diameter. After boring, oversized pistons must be installed in the engine.
The machine shop will use a large boring bar (machine tool) to cut a thin layer of metal off the cylinder walls. Normally a cylinder block is bored in increments of 0.010" or 0.25 mm.
Overbore limit, typically 0.030" - 0.060", is the largest possible diameter increase to which a cylinder can be bored. It is specified by the engine manufacturer and can vary with block design. If the overbore limit is exceeded, the cylinder wall can become too thin. The wall can distort or crack when used from combustion heat or pressure
Oversized pistons and rings are required to fit a cylinder block that has had its cylinders bored out. The pistons must match the oversize of the cylinders.
Boring cylinders and installing oversized pistons will help restore the engine to like-new condition. New pistons and rings will operate on freshly machined cylinder surfaces, providing excellent ring sealing and renewed life to the engine.
Cylinder sleeving involves machining one or more of the cylinders oversize and pressing in a cylinder liner. Sleeving is needed when the damage to the cylinder wall is too severe to correct with boring.
Sleeving allows the bad cylinder to be restored to its original diameter. The same size pistons can be reused. If only one cylinder is damaged, for instance, all of the other pistons and cylinders may be good and usable. This provides savings in time and money.
Using replaceable cylinder liners can save time and costly machine work. First, determine the type of liners, wet or dry, that are used in the unit being rebuilt. Dry liners do not require a water seal and can simply be pulled out and the new liner pressed into place. Wet liners have grooves cut into them for fitting O-ring seals to prevent water leakage into the crankcase.
When installing the wet type of liners, use care to prevent damage to the O-ring seals.
Before installing press-fit dry sleeves, inspect and measure the cylinder diameters. If the cylinders are distorted, the block must be re-bored to accept oversized liners. Otherwise, the sleeves may conform to the distorted cylinders. In some cases, air pockets will form between the sleeves and the block, causing localized hot spots that often result in a breakdown of the contact surfaces. Check the cylinder sleeve's outside diameter with an outside micrometer. Before installing the sleeve, heat the block and pack the sleeve with dry ice. After removing the dry ice from the sleeve, coat it with an appropriate lubricant and press or drive it into the cylinder until it touches the lip at the bottom of the cylinder. To drive the sleeve down into the cylinder, use a cylinder driver or hammer and a sleeve driver.
When installed properly, the sleeve will extend slightly above the deck of the block. A boring bar with a face tool installed in the cutter head is used to machine the top of the sleeve flush with the deck. After installation, the sleeve should be bored to the desired size, just like any other cylinder. The sleeved cylinder is then chamfered and washed.
The important factors to consider when installing a dry liner with a flange include the following:
Before installing a wet cylinder liner, clean all deposits from under the liner's flange and the mating counterbore in the cylinder block. The liner must rest flatly in the counterbore to prevent distortion; if the flange surface is uneven, re-machine it.
Clean the lower sealing surfaces in the block and on the liner to prevent coolant leaks when the liner is installed. Place new seals on the liner and lubricate both the seals and mating surfaces in the block. Check to be sure the seals are not twisted or crimped. A roll or twist increases the density of the seal in the area of the twist. The dense area produced by the twist creates a hard spot that attracts heat. A twisted seal can also distort the liner, reduce the piston operating clearance, and promote failure.
To remove twists from seals, insert the shaft of a small screwdriver under the seal at a right angle to the seal groove and rotate the screwdriver around the liner three or four times. On O-rings, a parting line is usually visible in the center of the outside diameter of the seal. This line should be parallel to the groove when the twist is eliminated.
Insert the liner into the cylinder until it contacts the crevice seal, place the palms of your hands on the upper end of the liner, and push it downward with quick, even pressure.
To continue the installation, tap the liner near its inside diameter with a large, soft-faced hammer. Tap alternately from side to side, gradually working around the entire circumference of the liner. When the liner is 1.5" above the deck, blow some compressed air into the counterbore to remove any material that may have accumulated. Deposits in the counterbore can cause distortion of the liner. When the counterbore is clean, continue to drive the liner fully into the block.
After the liner is seated properly, clean the liner flange with a brass wire brush to ensure an accurate measurement of the flange height above the block deck. Check the flange height above the block deck using a sled dial arrangement.
If a cylinder block deck is resurfaced, the cylinder block counterbore depth must be recut to specifications. If the cylinder block deck has not been resurfaced, but there is excessive pitting or erosion of the cylinder block counterbore, recut the counter bore as required.
Shims are also available to re-establish the correct flange height. Shims are manufactured in various thicknesses. Use only the number of shims necessary to obtain the correct liner height. Check the shim's thickness with a micrometer. If more than one shim is required, position the thickest shim on the bottom of the counterbore. Position the shim(s) into the cylinder liner counterbore.
When installation is complete, carefully clean the cylinder liner as previously described. Check the liner diameter using a dial bore gauge. Take readings at 180° apart at two levels.
After installing a new wet liner, check the top of the block and the cylinder heads for flatness. Check the counterbore depths for waviness and variations. Be sure counterbores are clean and free of dirt and carbon. Also measure the liner flange height.
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When service is required on pistons and rings, they must first be removed from the engine. Where removal is to be from the top of the cylinder block, take the cylinder head off and examine the cylinder for wear. If the cylinder is worn, there will be a ridge at the upper limit of the top ring travel. Remove this ridge. If not removed, it will damage the piston and rings as they are forced out of the top of the cylinder.
To remove this ridge, use a reamer. Before placing the ridge reamer in the cylinder, be sure the piston has been placed at BDC. Stuff rags into the cylinder to protect the piston and piston rings from metal shavings during the reaming operation. Be sure to adjust the cutters to the correct depth of cut. After the reaming operation is complete, remove the rags and wipe the cylinder wall clean. Repeat the operation for each cylinder.
Before you can detach the connecting rods from the crankshaft, you must remove the oil pan. With the cylinder head and oil pan off, crank the engine so that the piston of the No.1 cylinder is near BDC. Examine the piston rod and rod cap for identifying marks, and, if none can be seen, mark them with numbering dies to ensure replacing them in the same cylinders from which they were removed. Remove the rod nuts and cap them with a wrench, and slide the rod and piston assembly up into the cylinder away from the crankshaft and out of the cylinder. Place the assembly on a workbench and repeat this operation until all piston and rod assemblies have been removed.
After the piston assemblies are removed from the engine and disassembled, clean them with a detergent solution or an approved solvent, and blown dry with compressed air. To remove stubborn carbon deposits, use a chemical solvent that does not harm the piston's surface coating. Never scrape the piston with a groove cleaner or a broken piston ring. Hard scraping will scratch the machined surfaces of the piston. The two best methods of cleaning pistons are chemical and bead cleaning.
After the pistons have been cleaned, lightly engrave the proper bore number on the bottom of the piston to ensure the piston is reinstalled in the proper cylinder.
Never use gasoline to clean parts. Gasoline is highly volatile, and the slightest spark or flame could ignite the fumes causing a deadly fire.
Use extreme care when using an air gun. Wear goggles and avoid aiming the gun at yourself or anyone else. If air is injected into the bloodstream, death could result.
Pistons are made of aluminum, which is prone to wear and damage. It is critical that each piston be checked thoroughly. Look for cracked skirts, worn ring grooves, cracked ring lands, worn pin bores, and any other wear or damage. Trouble that could affect piston performance and service life must be identified.
After the piston has been cleaned, use a large outside micrometer to measure piston wear. Compare the readings to the specifications to determine the amount of wear that has occurred. The specifications will detail the measurements and allowable clearances as well as the maximum and minimum allowable piston and cylinder wall taper. Most of the pistons encountered will be cam ground pistons. A piston has a major axis and minor axis. The major axis is at right angles to the piston pin, and the minor axis is parallel to the piston pin. Both trunk and crosshead pistons are frequently cam ground, so that they are slightly elliptical rather than perfectly round. A cam ground piston is constructed so that its diameter parallel to the minor axis is less than that along the major axis. This is done to compensate for the different rates of thermal expansion that occur in the piston's minor and major axis diameters (Figure 32). Since the pin bore boss causes the minor axis to be thicker than the major axis, it expands more as it heats up. Therefore, the diameter parallel to this axis is smaller to allow for greater expansion.
Figure 32 -Cam ground pistons are elliptical in shape.
Piston size is measured on the skirt, just below the piston pin hole. Adjust the micrometer for a slight drag it is pulled over the piston. If the piston wear exceeds specifications, replace or knurl the piston(s).
Piston taper is measured by comparing piston diameter at the top, even with the pin hole, of the skirt-to-piston diameter at the bottom of the skirt. The difference between the two measurements equals piston taper. If taper is not within the service manual limits, replace or knurl the piston.
Piston knurling can be used to increase the diameter of the skirt a few thousandths of an inch. Knurling makes dents in the skirts, pushing up the metal next to the dents. This increases the piston diameter.
To find the piston clearance, subtract the piston diameter from the cylinder diameter. The difference between the cylinder diameter measurement and the piston diameter measurement will equal piston clearance.
Average piston-to-cylinder clearance is about 0.001". Since specifications vary, always refer to the service manual.
Another way of measuring piston clearance is using a long, flat feeler gauge which is placed on the piston skirt; then the gauge and piston are pushed into the cylinder. A spring scale is used to pull the feeler gauge out of the cylinder. When the spring scale reading equals specifications, the size of the feeler gauge equals piston clearance.
When piston-to-cylinder clearance is excessive, do the following:
Depending on the type and make of engine, the piston pin may either be free-floating or press-fit:
Figure 33 - A free-floating piston.
During piston and rod service, check the pin clearance on both free-floating and press- fit pins. Check the pin-to-connecting-rod fit on the free-floating piston pins. With press-fit pins, the piston pin should be locked tightly in the connecting rod.
To check excessive piston pin clearance, clamp the connecting rod I-beam lightly in a vise and, holding the piston straight up, rock the piston. If play can be detected, the pin, rod bushing, or piston bore is worn. A small telescoping gauge and an outside micrometer should be used to determine exact clearance after pin removal.
To remove a free-floating pin from the piston, use snap ring pliers to compress and fit out the snap rings on each end of the pin. Then, push the pin out of the piston with your thumb. In some cases, a brass drift and light hammer blows may be needed to drive the pin from the piston.
When the pin is worn, it should be replaced. If the pin bore in the piston measures larger than specifications, the piston must generally be replaced. In some cases, the pin bores can be reamed larger and oversized piston pins can be used.
To remove a pressed-in piston pin, you will need to use a press and a driver setup.
When pressing out piston pins, wear eye protection and make sure the piston is secured.
Measure pin and pin bore wear. Compare your measurements to the specifications and replace or repair parts as needed. If needed, send new pistons and pins to the machine shop for fitting.
Before installing a piston pin, make sure the piston is facing in the right direction in relation to the connecting rod. Normally, a piston will have a marking on its head that should point toward the front of the engine.
One edge of the connecting rod's big end bore end must face the outside of the crank journal during piston installation. The rod may also have an oil spray hole or rod numbers that must face in a specified direction. Check the vehicle's shop manual for directions.
To start a pressed-in piston-pin, tap it into the piston bore with a brass hammer. Then use a press to force the pin into the piston. The connecting rod small end must be centered on the pin.
After pushing a free-floating piston pin into the piston, install the snap rings to secure the pin. Make sure the snap rings are fully seated in their grooves.
Before installing piston rings, the piston should be cleaned and the ring grooves checked for carbon or dirt deposits. Rings must be installed with the top side up to provide proper oil control. Refer to the service manual for the proper instructions on the piston rings being installed. Generally, each new ring set contains an instruction sheet.
Install the compression rings on the piston and stagger the ring gaps. To avoid overstressing the rings during installation, do not spread the rings more than needed to slip them onto the piston. Using a ring expander will make this task much easier (Figure 34).
Figure 34 - A piston ring expander.
The use of a ring expander prevents the possibility of overspreading the rings. Select the correct ring for the ring groove. Use the ring expander tool to expand the piston ring, and then place the ring and tool over the piston. Set the ring in the groove and release the tool. Double check the installation to make sure that the ring is in the right groove and is not upside down.
Generally, the oil control rings are installed with the scraping edge down. Make sure that the ring ends do not overlap. Install the bottom ring with the gap positioned 45° from the top oil ring gap. A common method for checking the ring gap and clearance is shown in Figure 35. To determine whether the ring has the proper end gap, place it in the cylinder, pushing it about halfway down in the cylinder bore. With the ring square with the cylinder bore (use a piston to straighten the ring in the cylinder), measure the gap between the ring ends with the feeler gauge. If the gap is less than the minimum specified, remove the ring and dress ends with a fine-cut mill file until correct clearance is obtained
Figure 35 - Checking piston ring gap.
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Large engines are expensive items. Repairs, as evidenced by the preceding overhaul procedures, are costly and time consuming. The engine must be properly run-in before being released for use. Because of this, to get the most out of the newly overhauled engine, use proper initial startup and run-in procedures. The durability and service life of a rebuilt engine are directly affected by the quality of the run-in procedures.
An engine can be run-in using one of three methods:
The exact procedure for the run-in will vary depending on the type of method used, but in all cases the rebuilt engine must be properly prepared before starting for the first time.
Regardless of the method used for engine run-in, the engine must be properly prepared before starting for the first time. The following may be used as a general guide:
Lubrication System: The lubricating oil film of the rotating parts and bearings of an overhauled engine is usually insufficient for proper lubrication when the engine is started for the first time after an overhaul. Install new oil filters and fill a pressure prelubricator (usually an electrically or pneumatically actuated oil pump) with the OEM recommended oil and connect the supply line to the main oil gallery. Prime the engine lubrication system with sufficient oil. There are various points on the engine into which the pressure line may be tapped, but if no other is apparent, the oil gauge line may be disconnected and the pressure tank applied at that point. Remove the oil level dipstick, and check the crankcase level. Add oil, if necessary, to bring it to the full mark on the dipstick.
DO NOT OVERFILL! Excessive overfilling of the lubrication system will cause damage.
When using a prelubricator, it is unnecessary to prime the oil filters. Some OEMs prefer that the oil filters are never primed due to the risk of contaminating the oil during the procedure. When priming filters, pour the oil into the inlet side of the filter and never into the outlet side.
Turbocharger: Disconnect the turbocharger oil inlet line and pour approximately one pint of clean engine oil into the turbo, ensuring that the bearings are lubricated for the startup. Reconnect the oil line.
Air Intake System: Check the integrity of the air intake system, checking all the hose clamps, support clamps, piping, charge air cleaner, and the air cleaner element(s). Always replace the air cleaner element after an engine overhaul.
Cooling System: Fit a new coolant filter and if required, separate conditioning additives. Fill the cooling system with the recommended coolant mixture. Ensure that all or at least most air is purged from the cooling system. Remove a plug from the water manifold during filling to facilitate air to escape.
Fuel System: Install new fuel filters, priming them as required, with the correct grade of filtered fuel. Next, prime the fuel system by actuating a hand pump or external priming pump.
Avoid priming a fuel system by charging the fuel tanks with compressed air. This practice can be particularly dangerous when ambient temperatures are high.
Electrical System: Ensure that the batteries hold a proper level of charge. This is especially critical with some electronically managed engines in which the engine/electronic control module requires a specific minimum operating voltage.
Recheck all mounting bolts, and be sure that all belt drives are in place and proper tension. Check around to ensure there are no loose objects or items lying around to get caught into moving components.
Upon starting the newly overhauled engine, if you do not observe any oil pressure in the first 10 to 15 seconds, shut the engine down and find the cause. If oil pressure is observed, allow the engine to warm up at an idle. Do NOT load the engine before it is fully warmed up. During this warm-up period, check for any leaks and listen for any abnormal noises that could indicate trouble. After the warm-up period, shut the engine down and check all fluid levels, repair any leaks, and re-torque. In the case of any bolts, tighten as required.
After doing a thorough re-inspection of the vehicle, re-start the engine and prepare for a road test. In the case of road type vehicles such as pickups, tractor-trailers, and dump trucks, operate any auxiliary components such as a dump bed and hook a trailer up to the tractor before heading out on the road.
Once on the road, bring the vehicle up to speed and test the performance through the torque rise profile. Operate through all the operating gears and loads as feasible. A road test of at least 30 minutes after the specified engine operating temperature has been reached is usually adequate to break in a rebuilt engine. When diagnosing engine malfunctions, the road test duration will depend on the data revealed during the route.
The most probable time for a newly overhauled engine to malfunction is during its initial run-in and break-in period. Therefore, it is absolutely necessary that when these units are returned to service, they are done so with special instructions to the dispatcher and yard boss; for instance, only light loads for the first 500 miles/50 hours, and watch all fluid levels, temperatures, and pressures carefully. Last, ensure that the unit is brought into the shop after the break-in period for an oil and filter change. The unit is now ready for full service.
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This manual presented information on engine troubleshooting and overhaul that you, as supervisor, will be able to use to help your mechanics do an efficient and competent job maintaining the NCF's automotive and heavy equipment engines. This information included a discussion of the different types of horsepower, graphs and diagrams used to describe engine performance, causes of power loss and failure, and care and maintenance of engine gauges and test equipment. This manual also provided troubleshooting guidelines for various engine components such as valves, camshafts, crankshafts, cylinders, and pistons. Finally, you were given information on performing a pre-start check of all systems and components, as well as a run-in to make sure the vehicle/equipment is ready for use.
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1. Observing the highest degree of cleanliness in handling engine parts during overhaul is an elementary principle that applies to engine troubleshooting and overhaul.
2. The amount of work produced on the power stroke can be determined by what formula?
3. Horsepower was the "invention" of .
4. The formula for 1 horsepower is .
5. Air in its compressed state and contained in a reservoir has the potential for creating motion. What type of energy is this?
6. In a diesel engine, force is represented by what type of pressure?
7. Torque very simply is effort.
8. The formula for indicated horsepower is P x L x A x N x C / 33,000.
9. A four-stroke-cycle, eight-cylinder diesel engine with a bore of 6.0" and stroke of 6.5" is run at a speed of 2,500 rpm. The mean effective pressure obtained from an indicator diagram is specified as 240 psi. Calculate the indicated horsepower of the engine.
10. Brake horsepower is measures at what component?
11. A dynamometer is used to accurately measure torque.
12. What piece of test equipment enables the mechanic to fully load test the engine before it is reinstalled?
13. The chassis dynamometer allows for a controlled vehicle .
14. The difference between indicated horsepower and brake horsepower is referred to as horsepower.
Refer to the figure below when answering question 15.
15. Engine torque increases steadily in which of the following speed ranges?
Refer to the figure below when answering question 16.
16. In which of the following speed ranges does torque NOT fall while horsepower rises??
Refer to the figure below when answering questions 17 and 18.
17. The intake valve of a four-stroke-cycle diesel engine opens during which event?
18. What stroke of a four-stroke-cycle diesel engine begins slightly before TDC, continues through BDC, and ends during the next upstoke of the piston?
Refer to the figure below when answering question 19.
19. What is the relationship between fuel injection timing and piston position?
20. Which malfunction can cause an engine to lose power?
21. You do not have to consider the effects of altitude when analyzing engine power.
22. What are two main causes of excessive oil consumption?
23. Valve and tappet noise that is intermittent rapping noise that appears and disappears every few seconds indicates what condition?
24. Connecting rods make two distinct knocks. One is a metallic clack by the gudgeon pin end and the other is a loud, deep toned knock of the .
25. The sound you may hear during acceleration as high-pitched rattling or clicking in the upper part of the cylinder is .
26. What is a cylinder compression tester used to measure?
27. The compression ratio is determined by comparing the amount of space in the cylinder when the piston is at top of its travel to the space available when the piston is at its lowest point of travel.
28. The vacuum gauge provides a means of testing all these except which one?
29. The vacuum gauge is considered one of the most useful and versatile tools or test devices in the mechanic's TOA. If the gauge is reading 15-22 inches with the needle holding steady, what does this indicate?
30. Using the vacuum gauge, when you observe the needle swing back and forth over 4-6 inches, what does this indicate?
31. A device for introducing compressed air into the cylinder of an engine can be made by removing the insulator from an old spark plug and welding a pneumatic valve stem to the threaded end of the plug.
32. When using compressed air to test an engine cylinder for leakage, you notice air bubbles in the radiator coolant. The bubbles indicate that air is probably being released by what means?
33. Unusual noise at the cam followers, pushrods, and rocker arms is an indication of
34. Valve breakage can happen to either the intake or exhaust valves are destructive causing damage to the .
35. When valves are adjusted with too little clearance, they will .
36. In order to remove the valves from the cylinder head, you must first remove what parts?
37. Grinding by removing metal from the valve face will increase stem height. This affects the spring tension and .
38. The clearance between the valve stem and guide is important for a number of reasons. Which of these is NOT one of them?
39. What is the fastest method to restore the inside of a worn integral valve guide?
40. What method of valve guide restoration is considered superior to knurling by allowing oversized valves to be installed?
41. Threaded bronze inserts provide better lubrication, excellent wear qualities, and tight clearance, and they are fairly inexpensive and easily installed.
42. During the process of grinding valve seats, a valve seat grinder is kept concentric with the valve guide by what means?
43. One method of checking the valve seating is to coat the valve face lightly with Prussian blue and twist the valve one-quarter turn in its seat. How can you tell whether the valve seat is concentric with the valve guide?
44. When inserting a new valve seat, you should use which technique?
45. Which is not a normal check done on valve springs?
46. When you install a new camshaft, it is generally acceptable to reinstall the old lifters because the new cams will conform to the old lifters.
47. To indicate the end of a leak-down rate test on a hydraulic valve lifter, what action takes place as the valve seats?
48. In the installation of new camshaft bearings, it is important that you take which step?
49. Journals must be ground if they are tapered or out-of-round in excess of what measurement, in inches?
50. After the crankshaft is installed and the bearing caps are put in place, what action should you take?
51. Crankshaft end play will become excessive if the is/are worn, producing a sharp, irregular knock.
52. It is best to store a crankshaft by which method?
53. What causes the cylinder to taper?
54. In order to true worn cylinders and break the glaze on the cylinders you will use what method?
55. What type honing is recommended when the cylinder is in good condition?
56. Cylinder boring is required when the cylinder has which condition?
57. If only one or two cylinders need to be bored, what should you do to install the same size pistons?
58. After boring the cylinder you can fit it with a dry liner. How should you install the liner?
59. A liner that is loose will not make proper contact with the cylinder block and will result in .
60. On wet cylinder liners, what is used to re-establish the correct flange height?
61. For what reason should a cylinder ridge be removed on an engine being overhauled?
62. Scraping the sides of the piston during cleaning may leave scratches that can cause excessive cylinder wall wear.
63. A cam ground piston is slightly elliptical in order to compensate for .
64. To start a pressed-in piston pin, tap it into the piston bore with a 5-lb. hammer to ensure proper pressure.
65. Although installing piston rings is a fairly simple process, make certain that the ring ends do not .
66. Piston ring clearance is measured at what position on the piston?
67. Before starting a newly overhauled engine, you should inspect which system?
68. Upon starting a newly overhauled engine, you must shut down the engine if no oil pressure is observed in what maximum number of seconds?
69. A newly rebuilt engine should be run with light loading for at least (a) how many hours and (b) what number of miles?
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