The construction of an engine varies little, regardless of size and design. The intended use of the engine determines its size and design, and the temperature at which the engine will operate determines the type of metal it will be built from.
To simplify the service parts and servicing procedures in the field, the current trend in engine construction and design is toward engine families. Typically, there are several types of engines because of the many jobs to be done; however, the service and service parts problem are simplified by designing engines so they are closely related in cylinder size, valve arrangement, and so forth. For example, the GM series 71 engines can be obtained in two-, three-, four-, and six-cylinder in-line models. GM V-type engines come in 6-, 8-, 12-, and 16-cylinder models. These engines are designed in such a way that many of the internal parts can be used on any of the models.
The stationary parts of an engine include the cylinder block and cylinders, the cylinder head or heads, and the exhaust and intake manifolds. These parts furnish the framework of the engine. All movable parts are attached to or fitted into this framework.
The cylinder block is the basic frame of a liquid-cooled engine whether it is in-line, horizontally opposed, or V-type. The cylinder block is a solid casting made of cast iron or aluminum that contains the crankcase, the cylinders, the coolant passages, the lubricating passages, and, in the case of flathead engines, the valves seats, the ports, and the guides.
The cylinder block is a one-piece casting usually made of an iron alloy that contains nickel and molybdenum. This is the best overall material for cylinder blocks. It provides excellent wearing qualities and low material and production cost, and it changes dimensions only minimally when heated. Another material used for cylinder blocks, although not extensively, is aluminum. Aluminum is used whenever weight is a consideration. However, it is NOT practical to use for the following reasons:
• Aluminum is more expensive than cast iron.
• Aluminum is not as strong as cast iron.
• Because of its softness, it cannot be used on any surface of the block that is subject to wear. This necessitates the pressing, or casting, of steel sleeves into the cylinder bores. Threaded holes must also be deeper. This introduces extra design considerations and increases production costs.
• Aluminum has a much higher expansion rate than iron when heated. This creates problems with maintaining tolerances.
The cylinders are bored right into the block. A good cylinder must be round, not varying in diameter by more than approximately 0.0005 inch (0.012 mm). The diameter of the cylinder must be uniform throughout its entire length. During normal engine operation, cylinder walls wear out-of-round, or they may become cracked and scored if not lubricated or cooled properly. The cylinders on an air-cooled engine are separate from the crankcase. They are made of forged steel. This material is most suitable for cylinders because of its excellent wearing qualities and its ability to withstand the high temperatures that air-cooled cylinders obtain. The cylinders have rows of deep fins cast into them to dissipate engine heat. The cylinders are commonly mounted by securing the cylinder head to the crankcase with long studs and sandwiching the cylinders between the two. Another way of mounting the cylinders is to bolt them to the crankcase, and then secure the heads to the cylinders.
Cylinder sleeves, or liners, are metal pipe- shaped inserts that fit into the cylinder block. They act as cylinder walls for the piston to slide up and down on.
Cast iron sleeves are commonly used in aluminum cylinder blocks. Sleeves can also be installed to repair badly damaged cylinder walls in cast iron blocks. There are two basic types of cylinder sleeves, dry and wet.
A dry sleeve (Figure 2-1), presses into a cylinder that has been bored or machined oversize. A dry sleeve is relatively thin and is not exposed to engine coolant. The outside of a dry sleeve touches the walls of the cylinder block. This provides support for the sleeve.
Figure 2-1 – Dry cylinder sleeve.
When a cylinder becomes badly worn or is damaged, a dry sleeve can be installed. The original cylinder must be bored almost as large as the outside of the sleeve. Then, the sleeve is pressed into the oversized hole. Next, the inside of the sleeve is machined to the original bore diameter. This allows the use of the original piston size.
A wet sleeve (Figure 2-2), is exposed to the engine coolant. It must withstand combustion pressure and heat without the added support of the cylinder block. Therefore, it must be thicker than a dry sleeve.
Figure 2-2 – Wet cylinder sleeve.
A wet sleeve will generally have a flange at the top. When the head is installed, the clamping action pushes down on the sleeve and holds it in position. The cylinder head gasket keeps the top of the sleeve from leaking. A rubber or copper O-ring is used at the bottom of a wet sleeve to prevent coolant leakage into the crankcase. The O- ring seal is pinched between the block and the sleeve to form a leak-proof joint.
Many vehicles use aluminum cylinder blocks with cast iron wet sleeves. The light aluminum block reduces weight for increased fuel economy. The cast iron sleeves wear very well, increasing engine service life.
Most cylinder sleeve casualties are directly related to a lack of maintenance or improper operating procedures. Figure 2-3 shows two common types of cylinder sleeve casualties: cracks and scoring. Both types of casualties require replacement of the sleeve.
Figure 2-3 – Sleeve casualty.
See Cylinder Sleeve (above).
The cylinder block also provides the foundation for the cooling and lubricating systems. The cylinders of a liquid-cooled engine are surrounded by interconnecting passages cast in the block. Collectively, these passages form the water jacket that allows the circulation of coolant through the cylinder block and the cylinder head to carry off excessive heat created by combustion.
The water jacket is accessible through holes machined in the head and block to allow removal of the material used for casting of the cylinder block. These holes are called core holes and are sealed by core hole plugs (freeze plugs). These plugs are of two types: cup and disk. Figure2-4 shows a typical location of these plugs.
Figure 2-4 – Core hole plugs
The crankcase (Figure 2-5), is that part of the cylinder block below the cylinders. It supports and encloses the crankshaft and provides a reservoir for lubricating oil.
Figure 2-5 – Crankcase.
The crankcase also has mounting brackets to support the entire engine on the vehicle frame. These brackets are either an integral part of the crankcase or are bolted to it in such a way that they support the engine at three or four points. These points are cushioned by rubber mounts that insulate the frame and body of the vehicle from engine vibration. This prevents damage to engine supports and the transmission.
The crankcase shown in Figure 2-6 is the basic foundation of all air-cooled engines. It is made as a one- or two-piece casting that supports the crankshaft, provides the mounting surface for the cylinders and the oil pump, and has the lubrication passages cast into it. It is made of aluminum since it needs the ability to dissipate large amounts
of heat. On air-cooled engines, the oil pan usually is made of cast aluminum and is covered with cooling fins. The oil pan on an air-cooled engine plays a key role in the removal of waste heat from the engine through its lubricating oil.
Figure 2-6 — Air-cooled crankcase.
The cylinder head (Figure 2-7), bolts to the deck of the cylinder block. It covers and encloses the top of the cylinders. Combustion chambers, small pockets formed in the cylinder heads where combustion occurs, are located directly over the cylinders. Spark plugs (gasoline engine) or injectors (diesel engine) protrude through holes into the combustion chambers.
Intake and exhaust ports are cast into the cylinder head. The intake ports route air (diesel engine) or air and fuel (gasoline engine) into the combustion chambers. The exhaust port routes burned gases out of the combustion chamber.
Figure 2-7 – Cylinder head.
Valve guides are small holes machined through the cylinder head for the valves. The valves fit into and slide in these guides.
Valve seats are round, machined surfaces in the combustion chamber port openings. When a valve is closed, it seals against the valve seat.
The cylinder head is built to conform to the arrangement of the valves: L-head, I-head, or others. Cylinder heads on liquid-cooled engines have been made almost exclusively from cast iron until recent years. Because weight has become an important consideration, a large percentage of cylinder heads now are being made from aluminum.
The cylinder heads on air-cooled engines are made exclusively from aluminum because aluminum conducts heat approximately three times as fast as cast iron. This is a critical consideration with air cooling.
In liquid-cooled engines, the cylinder head is bolted to the top of the cylinder block to close the upper end of the cylinders and, in air-cooled engines, the cylinder heads are bolted to the top of the cylinders.
In a liquid-cooled engine, a cylinder head also contains passages, matching those of the cylinder block, that allow coolant to circulate in the head. These water jackets are for cooling spark plug openings, valve pockets, and part of the combustion chamber. In this type of cylinder head, the water jackets must be large enough to cool not only the top of the combustion chamber but also the valve seats, valves, and valve-operating mechanisms.
The cylinder heads are sealed to the cylinder block to prevent gases from escaping. This is accomplished on liquid-cooled engines by the use of a head gasket. In an air- cooled engine, cylinder heads are sealed to the tops of the cylinders by soft metal rings. The lubrication system feeds oil to the heads through the pushrods.
The exhaust manifold (Figure 2-8), connects all of the engine cylinders to the rest of the exhaust system. On L-head engines, the exhaust manifold bolts to the side of the engine block, whereas on overhead-valve engines, it bolts to the side of the cylinder head. It is made of cast iron, lightweight aluminum, or stainless steel tubing. If the exhaust manifold is made properly, it can create a scavenging action that causes all of the cylinders to help each other get rid of the gases. Back pressure (the force that the pistons must exert to push out the exhaust gases) can be reduced by making the manifold with smooth walls and without sharp bends.
Figure 2-8 – Exhaust manifold.
Exhaust manifolds on vehicles today are constantly changing in design to allow the use of various types of emission controls. Each of these factors is taken into consideration when the exhaust manifold is designed, and the best possible manifold is manufactured to fit into the confines of the engine compartment.
Figure 2-9 – Intake manifold.
The intake manifold on an L-head engine is bolted to the block, whereas the overhead- valve engine has the intake manifold bolted to the side of the cylinder head.
Intake manifolds can be designed to provide optimum performance for a given speed range by varying the length of the passages. The inertia of the moving intake mixture causes it to bounce back and forth in the intake manifold passage from the end of one intake stroke to the beginning of the next intake stroke. If the passage is the proper length so the next intake stroke is just beginning as the mixture is rebounding, the inertia of the mixture causes it to ram itself into the cylinder. This increases the volumetric efficiency of the engine in the designated speed range. It should be noted that the ram manifold serves no purpose outside its designated speed range.
As stated earlier, providing controlled heat for the incoming mixture is very important for good performance. The heating of the mixture may be accomplished by doing one or both of the following:
• Directing a portion of the exhaust through a passage in the intake manifold. The heat from the exhaust transfers and heats the mixture. The amount of exhaust that is diverted into the intake manifold heat passage is controlled by the manifold heat control valve.
• Directing the engine coolant, which is heated by the engine, through the intake manifold on its way to the radiator.
The lower part of the crankcase is the oil pan (Figure 2-10), which is bolted at the bottom. The oil pan is made of cast aluminum or pressed steel and holds the lubricating oil for the engine. Since the oil pan is the lowest part of the engine, it must be strong enough to withstand blows from flying stones and obstructions sticking up from the road surface.
Figure 3-10 – Oil pan.
Usually, a head gasket can be installed only one way. If it is installed backwards, coolant and oil passages may become blocked, causing serious problems. Markings usually indicate the front or top of the head gasket. The gasket may be marked with the word “top” or “front” or it may have a line to show installation direction. Metal dowels are often provided to align the head gasket.
Most modern, Teflon ®-coated, permanent-torque (retorquing is not needed after engine operation) cylinder head gaskets should be installed clean and dry. Sealer is not recommended. However, some head gaskets may require retorquing and sealer. When in doubt, refer to manufacturer’s instructions.
There are three types of manifold gaskets, the intake manifold, the exhaust manifold and a combination of the two. Each type of manifold gasket has its own sealing characteristics and problems. Therefore, be sure to follow the manufacturer’s instructions when installing them.
An oil pan gasket seals the joint between the oil pan and the bottom of the block. The oil pan gasket might also seal the bottom of the timing cover and the lower section of the rear main bearing cap.
The oil pan gasket must resist hot, thin engine oil. The gasket is made of several types of material. A commonly used material is synthetic rubber, known for its long-term sealing ability. It is tough and durable, and resists hot engine oil.
Figure 2-11 – Synthetic rubber
The synthetic rubber seal (Figure 2-11), is the most common type of oil seal. It is composed of a metal case used to retain its shape and maintain rigidity. A rubber element is bonded to the case, providing a sealing lip or lips against the rotating shaft. A coil spring, sometimes called a garter spring, is used to hold the rubber element around the shaft with a controlled force. This allows the seal to conform to minor shaft run out. Some synthetic rubber seals fit into bores mounted around the shaft. This type is generally a split design and does not require a metal case or garter spring. The internal pressure developed during operations forces the sealing lips tighter against the rotating shaft. This type of seal operates effectively only against fluid pressure from one direction.
The piston transfers the pressure of combustion to the connecting rod and crankshaft. It must also hold the piston rings and piston pin while operating in the cylinder. Pistons, (Figure 2-12) are normally cast or forged from an aluminum alloy. Cast pistons are relatively soft and are used in slow-speed, low-performance engines. Forged pistons are commonly used in today’s fuel- injected, turbocharged, and diesel engines. These engines expose the pistons to much higher stress loads, which could break cast aluminum pistons.
Figure 2-12 — Piston.
The piston must withstand incredible punishment under temperature extremes. The following are examples of conditions that a piston must withstand at normal highway speed:
The structural components of the pistons are the head, skirt, ring grooves, and lands (Figure 2-13); however, all pistons do not look like the typical one shown here. Some have differently shaped heads.
Figure 2-13 – Parts of a piston.
The piston head is the top of the piston and is exposed to the heat and pressure of combustion. This area must be thick enough to withstand these forces. It must also be shaped to match and work with the shape of the combustion chamber for complete combustion.
A piston skirt (Figure 2-14) is the side of the piston below the last ring. Without a skirt, the piston could tip and jam in the cylinder. A slipper skirt is produced when portions of the piston skirt below the piston ends are removed. The slipper skirt provides clearance between the piston and the crankshaft counterweights. This allows the piston to slide farther down in the cylinder without hitting the crankshaft. A straight skirt is flat across the bottom, a style no longer common in automotive engines.
Figure 2-14 – Piston skirts.
Piston ring grooves are slots machined in the piston for the piston rings. The upper two groves hold the compression rings. The lower piston groove holds the oil ring.
Piston oil holes in the bottom ring groove allow the oil to pass through the piston and onto the cylinder wall. The oil then drains back into the crankcase.
The piston ring lands are the areas between and above the ring grooves. They separate and support the piston rings as they slide on the cylinder.
The piston boss is a reinforced area around the piston pin hole. It must be strong enough to support the piston pin under severe loads.
A piston pin hole is machined through the pin boss for the piston pin. It is slightly larger than the piston pin.
The piston pin, also called the wrist pin, allows the piston to swing on the connecting rod. The pin fits through the hole in the piston and the connecting rod small end.
Piston clearance is the amount of space between the sides of the piston and the cylinder wall. Clearance allows a lubricating film of oil to form between the piston and the cylinder. It also allows for expansion when the piston heats up. The piston must always be free to slide up and down in the cylinder block.
A cam-ground piston (Figure 2-15) is slightly out-of-round when viewed from the top. The piston is machined a few thousandths of an inch larger in diameter perpendicular to the piston pin centerline.
Figure 2-15 – Cam-ground piston.
Cam grinding is done to compensate for different rates of piston expansion due to differences in metal wall thickness. As the piston is heated by combustion, the thicker area around the pin boss causes the piston to expand more parallel to the piston pin.
The oval-shaped piston becomes round when hot, and there is still enough clearance parallel to the piston pin.
The cold cam-ground piston has the correct piston-to-cylinder clearance. The unexpanded piston will not slap, flop sideways, and knock in the cylinder because of too much clearance. However, the cam-ground piston will not become too tight in the cylinder when heated to full operating temperature.
Piston taper is also used to maintain the correct piston-to-cylinder clearance. The top of the piston is machined slightly smaller than the bottom. Since the piston head gets hotter than the skirt, it expands more. The piston taper makes the piston almost equal in size at the top and bottom at operating temperature.
Piston shape generally refers to the contour of the piston head. Usually, a piston head is shaped to match the shape of the head. A flat top piston implies it has a flat head, that it is parallel to the deck of the head. Valve reliefs are cut into the head of these types of pistons.
A dished piston has a head that is sunken. This type of piston can be used to lower compression like in a supercharged engine.
A domed piston, or pop-up piston, has a head that is convex, or curved upward. This type of piston is normally used with a hemi-type cylinder head and some four-valve heads.
Diesel engine pistons have combustion cups machined into their heads. The combustion cup shape causes the fuel to move in a turbulent pattern as it enters the combustion chamber, allowing a more thorough mixture for efficient combustion. Two typical combustion cup designs are the sombrero cup and the turbulence cup.
The piston rings seal the clearance between the outside of the piston and cylinder wall. They must keep combustion pressure from entering the crankcase. They must also keep oil from entering the combustion chamber.
Most pistons use three rings, two upper compression rings and one oil ring on the bottom. The compression rings prevent blow by (combustion pressure leaking into the engine crankcase). The oil rings prevent oil from entering the combustion chamber.
Diesel engine pistons typically use a four-ring design because they are more prone to blow by. The four-ring piston has three compression rings from the top, followed by one oil control ring. This is due to the much higher pressures generated during the power stroke.
Connecting rods connect the pistons to the crankshaft to convert reciprocating motion into rotary motion. They must be strong enough to transmit the thrust of the pistons to the crankshaft and to withstand the internal forces of the directional changes of the pistons. The connecting rods (Figure 2-16) are in the form of an I-beam. This design gives the highest overall strength and lowest weight. They are made of forged steel but may also be made of aluminum in smaller engines.
Figure 2-16 – Connecting rod.
The upper end of the connecting rod is connected to the piston by the piston pin. The piston pin is locked in the pin bosses, or it floats in both piston and connecting rod. The upper hole of the connecting rod has a solid bearing (bushing) of bronze or similar material. As the lower end of the connecting rod revolves with the crankshaft, the upper end is forced to turn back and forth on the piston pin. Although the movement is slight, the bushing is necessary because the temperatures and pressures are high. If the piston pin is semi- floating, a bushing is not needed.
The lower hole in the connecting rod is split so it can be clamped around the crankshaft. The bottom part, or cap, is made of the same type of material as the rod and is attached by two or more bolts. The surface that bears on the crankshaft is generally a bearing material in the form of a split shell, although in a few cases it may be spun or die-cast in the inside of the rod and cap during manufacture. The two parts of the separate bearing are positioned in the rod and cap by dowel pins and projections or by a short brass screw. The shell may be of Babbitt metal that is die-cast on a backing of bronze or steel.
The connecting rod bearings are fed a constant supply of oil through a hole in the crankshaft journal. A hole in the upper bearing half feeds a passage in the connecting rod to provide oil to the piston pin.
Connecting rod numbers are used to assure a proper location of each connecting rod in the engine. They all assure that the rod cap is installed on the rod body correctly. When connecting rod caps are being manufactured, they are bolted to the connecting rods.
Then the lower end holes are machined in the rods. Since the holes may not be perfectly centered, rod caps must NOT be mixed up or turned around. If the cap is installed without the rod numbers in alignment, the bore will NOT be perfectly round. Connecting rod caps, crankshaft, and bearing damage will result.
In addition to the proper fit of the connecting rod bearings and the proper position of the connecting rod, the alignment of the rod itself must be considered. That is to say, the hole for the piston pin and the crankpin must be precisely parallel. EVERY connecting rod should be checked for proper alignment just before it is installed in the engine. Misalignment of connecting rods causes many hard to locate noises in the engine.
As the pistons collectively might be regarded as the heart of the engine, so the crankshaft (Figure 3-17) may be considered its backbone. The crankshaft is located in the bottom of the engine and is the part of the engine that transforms the reciprocating motion of the piston to rotary motion. It transmits power through the flywheel, the clutch, the transmission, and the differential to drive your vehicle.
Crankshafts are usually made of cast iron or forged steel. Forged steel crankshafts are needed for heavy-duty applications, such as turbocharged or diesel engines. A steel crankshaft is stiffer and stronger than a cast iron crankshaft. It will withstand greater forces without flexing, twisting or breaking.
Oil passages leading to the rod and main bearings are either cast or drilled in the crankshaft. Oil enters the crankshaft at the main bearings and passes through holes in the main journals. It then flows through passages in the crankshaft and out to the connecting rod bearings.
With an inline engine, only one connecting rod fastens to each rod journal. With a V- type engine, two connecting rods bolt to each rod journal. The amount of rod journal offset controls the stroke of the piston. The journal surfaces are precision machined and polished to very accurate tolerances. It is common to have reduced journal, or crankpin, diameters in order to reduce friction in the bearings.
A fully counterweighted crankshaft has weights formed opposite every crankpin. A partially counterweighted crankshaft only has weights formed on the center area. A fully counterweighted crankshaft will operate with less vibration than a partially counterweighted crankshaft.
Figure 2-17 — Crankshaft.
The crankshaft is supported in the crankcase and rotates in the main bearings (Figure 2-17). The connecting rods are supported on the crankshaft by the rod bearings.Crankshaft bearings are made as precision inserts that consist of a hard shell of steel or bronze with a thin lining of anti-frictional metal or bearing alloy. Bearings must be able to support the crankshaft rotation and deliver power stroke thrust under the most adverse conditions.
The crankshaft rotates in the main bearings located at both ends of the crankshaft and at certain intermediate points. The upper halves of the bearing fit right into the crankcase and the lower halves fit into the caps that hold the crankshaft in place (Figure 3-17). These bearings often are channeled for oil distribution and may be lubricated with crankcase oil by pressure through drilled passages or by splash. Some main bearings have an integral thrust face that eliminates crankshaft end play. To prevent the loss of oil, place the seals at both ends of the crankshaft where it extends through the crankcase. When replacing main bearings, tighten the bearing cap to the proper tension with a torque wrench and lock them in place with a cotter pin or safety wire after they are in place.
Vibration due to imbalance is an inherent problem with a crankshaft that is made with offset throws. The weight of the throws tends to make the crankshaft rotate elliptically. This is aggravated further by the weight of the piston and the connecting rod. To eliminate the problem, position the weights along the crankshaft, placing one weight 180 degrees away from each throw. They are called counterweights and are usually part of the crankshaft but may be a separate bolt on items on small engines.
The crankshaft has a tendency to bend slightly when subjected to tremendous thrust from the piston. This deflection of the rotating member causes vibration. This vibration due to deflection is minimized by heavy crankshaft construction and sufficient support along its length by bearings.
Torsional vibration occurs when the crankshaft twists because of the power stroke thrusts. It is caused by the cylinders farthest away from the crankshaft output. As these cylinders apply thrust to the crankshaft, it twists and the thrust decreases. The twisting and unwinding of the crankshaft produces a vibration. The use of a vibration damper at the end of the crankshaft opposite the output acts to absorb torsional vibration.
The camshaft is also part of the valve train and will be discussed later in this chapter.
A valve train is a series of parts used to open and close the intake and exhaust ports. A valve is a movable part that opens and closes a passageway. A camshaft controls the movement of the valves, causing them to open and close at the proper time. Springs are used to close the valves.
A high frequency movement resulting from twisting and untwisting of the crankshaft is called harmonic vibration. Each piston and rod assembly can exert over a ton of downward force on its journal. This can actually flex the crank throws in relation to each other. If you do not control the vibration, serious damage can occur. A vibration damper or harmonic balancer (Figure 3-18), is used to control this vibration. The damper also cuts load variation on the engine timing belt, chain, or gears so they last longer.
The vibration damper is a heavy wheel mounted on a rubber ring to control harmonic vibration. It consists of two metal rings, the outer inertia ring and the inner sleeve, separated by a ring of rubber. The balancer is keyed to the crankshaft snout. This makes the damper spin with the crankshaft. The inertia ring and the rubber ring set up a damping action on the crankshaft as it tries to twist and untwist. This deadens vibration action. There is also a dual-mass harmonic balancer which has one weight mounted on the outside of the crankshaft pulley and another on the inside. The extra rubber-mounted weight helps reduce vibration at high engine speeds.
Figure 2-18 – Vibration damper.
The flywheel (Figure 2-19) stores energy from the power strokes and smoothly delivers it to the drive train of the vehicle between the engine and the transmission. It releases this energy between power impulses, assuring fewer fluctuations in speed and smoother engine operation. The flywheel is mounted at the rear of the crankshaft near the rear main bearing. This is usually the longest and heaviest main bearing in the engine, as it must support the weight of the flywheel.
Figure 2-19– Flywheel.
The flywheel on large, low-speed engines is usually made of cast iron. This is desirable because the heavy weight of the cast iron helps the engine maintain a steady speed. Small, high-speed engines usually use a forged steel or forged aluminum flywheel for the following reasons:
On a vehicle with a manual transmission, the flywheel serves to mount the clutch. With a vehicle that is equipped with an automatic transmission, the flywheel supports the front of the torque converter. In some configurations, the flywheel is combined with the torque converter. The outer edge of the flywheel carries the ring gear, either integral with the flywheel or shrunk on. The ring gear is used to engage the drive gear on the starter motor for cranking the engine.
There are two valves for each cylinder in most engines—one intake and one exhaust. Since these valves operate at different times, it is necessary that a separate operating mechanism be provided for each valve. Valves are held closed by heavy springs and by compression in the combustion chamber. The purpose of the valve actuating mechanism is to overcome spring pressure and open the valve at the proper time. The valve actuating mechanism includes the engine camshaft, the camshaft followers (tappets), the pushrods, and the rocker arms.
The camshaft provides for the opening and closing of the engine valves. The camshaft (Figure 2-20) is enclosed in the engine block. It has eccentric lobes (cams) ground on it for each valve in the engine. As the camshaft rotates, the cam lobe moves up under the valve tappet, exerting an upward thrust through the tappet against the valve stem or the pushrod. This thrust overcomes the valve spring pressure as well as the gas pressure in the cylinder, causing the valve to open. When the lobe moves from under the tappet, the valve spring pressure reseats the valve.
Figure 2-20 – Camshaft.
On L-, F-, or I-head engines, the camshaft is located to one side and above the crankshaft, while in V-type engines, it is located directly above the crankshaft. On the overhead camshaft engine, the camshaft is located above the cylinder head.
The camshaft of a four-stroke-cycle engine turns at one half of engine speed. It is driven off the crankshaft through timing gears or a timing chain. (The system of camshaft drive is discussed later in this chapter.) In a two-stroke-cycle engine, the camshaft must turn at the same speed as the crankshaft, so each valve opens and closes once in each revolution of the engine.
In most cases, the camshaft does more than operate the valve mechanism. It may have external cams or gears that operate the fuel pumps, the fuel injectors, the ignition distributor, or the lubrication pump.
Camshafts are supported in the engine block by journals in bearings. Camshaft bearing journals are the largest machined surfaces on the shaft. The bearings are made of bronze and are bushings, rather than split bearings. The bushings are lubricated by oil circulating through drilled passages from the crankcase. The stresses on the camshaft are small; therefore, the bushings are not adjustable and require little attention. The camshaft bushings are replaced only when the engine requires a complete overhaul.
Camshaft followers are part of the valve actuating mechanism that contacts the camshaft. You will hear them called valve tappets or valve lifters. The bottom surface is hardened and machined to be compatible with the surface of the camshaft lobe. There are four types of followers—hydraulic, mechanical, roller and the OHC follower.
Hydraulic valve lifters (Figure 2-21) are common because they operate quietly by maintaining zero valve clearance. Zero valve clearance means that there is no space between valve train parts. With zero clearance, the valve train does not clatter when the engine is running. The hydraulic lifter adjusts automatically with temperature changes and part wear. During engine operation, oil pressure fills the inside of the hydraulic lifter with motor oil. The pressure pushes the lifter plunger up in its bore until all the play is out of the valve train. As the camshaft pushes on the lifter, the lifter check valve closes to seal oil inside the lifter. Since oil is not compressible, the lifter acts as a solid unit to open the valve.
Figure 2-21 – Hydraulic lifters.
Mechanical lifters (Figure 2-22), also called solid lifters, do not contain oil. They simply transfer cam lobe action to the push rod. Mechanical lifters are not self-adjusting and require periodic setting. A screw adjustment is normally provided at the rocker arm when solid lifters are used. Turning the adjustment screw down reduces any “play” in the valve train. Unscrewing, or backing off, the rocker arm adjustment increases clearance. A clattering or clicking noise is produced as the valves open and close.
Figure 2-22 – Mechanical lifters.
A roller lifter (Figure 2-23) has a small roller that rides on the camshaft lobe. This type of lifter can be either mechanical or hydraulic. The point where the lifter touches the camshaft is one of the highest friction points in the engine. The roller helps reduce this friction and wear. A roller lifter is also used to reduce frictional losses of power.
Figure 2-23 – Roller lifters.
An OHC follower (Figure 2-24) fits between the camshaft and valve. The follower slides up and down in a bore machined in the head. Either an adjusting screw in the follower or shims of different thicknesses can be used to adjust valve clearance.
Figure 2-24 – OHC lifters.
Each cylinder in a four-stroke-cycle engine must have one intake and one exhaust valve (Figure 225). The valve design that is commonly used is the poppet, a word derived from the popping action of the valve.
Figure 2-25 – Valves.
Construction and design considerations are very different for intake and exhaust valves. The difference is based on their temperature operating ranges. Intake valves are kept cool by the incoming intake mixture. Exhaust valves are subject to intense heat from the burnt gases that pass by it. The temperature of an exhaust valve can be in excess of 1300°F. Intake valves are made of nickel chromium alloy, whereas exhaust valves are made from silichrome alloy. In certain heavy-duty and most air-cooled engines, the exhaust valves are sodium-filled. During engine operation, the sodium inside the hollow valve melts. When the valve opens, the sodium splashes down into the valve head and collects heat. Then, when the valve closes, the sodium splashes up into the valve stem. Heat transfers out of the sodium into the stem, valve guide, and engine coolant. In this way, the valve is cooled. Sodium-filled valves are light and allow high engine rpm for prolonged periods.
In vehicles that use unleaded fuel, a stellite valve is preferred. A stellite valve has a special hard metal coating on its face. Lead additives in gasoline, other than increasing octane, act as a lubricant. The lead coats the valve face and seat to reduce wear. With unleaded fuel, the wear of the valve seat and valve face is accelerated. A stellite valve prevents this and prolongs valve service life.
Valve seats (Figure 2-26) are important, as they must match the face of the valve head to form a perfect seal. The seats are made so they are concentric with the valve guides, that is, the surface of the seat is an equal distance from the center of the guide all around. Although some earlier engines were designed with flat contact surface for the valve and valve seat, most are now designed with valve seat angles of 30 to 45 degrees. This angle helps prevent excessive accumulation of carbon on the contact surface of the seat—a condition that keeps the valve from closing properly. To further reduce carbon build up, there is an interference angle (usually 1 degree) between the valve and seat. In some cases, a small portion of the valve seat has an additional 15-degree angle ground into it to narrow the contact area of the valve face and seat. When you reduce the contact area, the pressure between the mating parts is increased, thereby forming a better seal.
Figure 2-26 – Valve seats.
The valve seats may be an integral part of the cylinder head or an insert pressed into the cylinder head. Valve seat inserts are commonly used in aluminum cylinder heads. Steel inserts are needed to withstand the extreme heat. When a valve seat insert is badly worn from grinding or pitting, it must be replaced.
Figure 2-27 – Valve guides.
The valve guides (Figure 2-27) are the parts that support the valves in the cylinder head. They are machined to fit a few thousandths of an inch clearance with a valve stem. This close clearance is important for the following reasons:
Valve guides may be cast integrally with the head, or they may be removable. Removable guides are press-fit into the cylinder head.
Servicing of valve guides is an important but often neglected part of a good valve job. The guide must be clean and in good condition before a good valve seat can be made. Valve guide wear is a common problem; it allows the valve to move sideways in its guide during operation. This can cause oil consumption (oil leaks past the valve seal and through the guide), burned valves (poor seat to valve face seal), or valve breakage.
There are several satisfactory methods of checking for valve guide wear. One is to slide the valve into its guide, pull it open approximately 1/2 inch, then try and wiggle the valve sideways. If the valve moves sideways in any direction, the guide or stem is worn. Another checking procedure involves the use of a small hole gauge to measure the inside of the guide and a micrometer to measure the valve stem; the difference in the readings is the clearance. Check the manufacturer's manual for the maximum allowable clearance. When the maximum clearance is exceeded, the valve guide needs further servicing before you proceed with the rest of the job.
Servicing procedures depend on whether the guide is of the integral or replaceable type. If it is the integral type, it must be reamed to a larger size and a valve with an oversize stem installed. But if it is replaceable, it should be removed and a new guide installed.
Valve seat service requires either replacement of the seat or reconditioning of the seat by grinding or cutting. Valve seat replacement is required when a valve seat is cracked, burned, or recessed (sunk) in the cylinder head. Normally, valve seats can be machined and returned to service.
To remove a replaceable pressed-in seat, split the old seat with a sharp chisel. Then pry out the old seat. New seat inserts should be chilled in dry ice for about 15 minutes to shrink them, so they can be driven into place easily. The seat expands when returned to room temperature, which locks the seat in place.
In most cases, the valve seats are not replaceable, so they must be ground. Before operating the valve seat grinding equipment in your shop, be sure to study the manufacturer’s manual for specific instructions. The following procedure is typical for grinding valve seats:
After grinding valve seats, it is recommended that you lap the contact surfaces of the valve and valve seat in order to check the location of the valve-to-seat contact point and to smooth the mating surfaces.
To lap the valve, dab grinding compound (abrasive paste) on the valve face. Install the valve into the cylinder head and rotate with a lapping stick (a wooden stick with a rubber plunger for holding the valve head). Rub your hands back and forth on the lapping stick to spin the valve on its seat. This rubs the grinding compound between the valve face and the seat. Remove the valve and check the contact point. A dull gray stripe around the seat and face of the valve indicates the valve-to-seat contact point. This helps you narrow or move the valve seat. A few manufacturers do NOT recommend valve lapping. Refer to the manufacturer’s service manual for details.
Make sure you clean all of the valve grinding compound off the valve and cylinder head.The compound can cause rapid part wear.
Another way to check valve-to-seat contact is by spreading a thin coat of prussian blue on the valve face or putting lead pencil marks on the valve seat. If, when turning the valve on its seat, you see an even deposit of coloring on the valve seat or the pencil lines are removed, the seating is perfect. Do NOT rotate the valve more than one-eighth turn, as a high spot could give a false indication if turned one full revolution.
The seat should touch near the center of the valve face with the correct contact width. Typically, an intake valve should have a valve-to-seat contact width of about 1/16 of an inch. An exhaust valve should have a valve-to seat contact width of approximately 3/32 of an inch. Check the manufacturer’s service manual for exact values.
When the valve seat does NOT touch the valve face properly (wrong width or location on the valve), regrind the seat using different angles, usually 15-degree and 60-degree stones. This is known as narrowing or positioning a valve.
To move the seat in and narrow it, grind the valve seat with a 15-degree stone. This removes metal from around the top of the seat. The seat face moves closer to the valve stem.
To move the seat out and narrow it, grind the valve seat with a 60-degree stone. This cuts away metal from the inner edge of the seat. The seat contact point moves toward the margin or outer edge of the valve.
After prolonged use, valve springs tend to weaken, lose tension, or even break. During engine service, always test valve springs to make sure they are usable. Valve springs should be tested for uniformity and strength. The three characteristics to check are valve spring squareness, valve spring free height, and valve spring tension.
Valve spring squareness is easily checked with a combination square. Place each 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 any spring that is not square.
Valve spring free height can also be measured with a combination square or a valve spring tester. Simply measure the length of each spring in normal uncompressed condition. If it is too long or too short, replace the spring.
Valve spring tension, or pressure, is measured by using a spring tester. Compress the spring to specification height and read the scale on the tester. Spring pressure must be within specifications. If the reading is too low, the spring has weakened and must be replaced.
Timing gears (Figure 2-28) are common in engines used for heavy-duty applications, such as taxi cabs or trucks. They are very dependable and long lasting. However, they are noisier than a chain or belt drive. Gears are primarily used for cam-in-block engines where the crankshaft is close to the camshaft.
Figure 2-28 – Timing gears.
Two timing gears are used to drive the engine camshaft. A crank gear is keyed to the crankshaft snout. It turns a cam gear on the end of the camshaft. The cam gear is twice the size of the crank gear. This results in the desired 2:1 reduction.
Timing marks on the two gears show the technician how to install the gears properly. The marks may be circles, indentations, or lines on the gears. The timing marks must line up for the camshaft to be in time with the crankshaft.
Bearings (Figure 2-29) are installed in an engine where there is relative motion between parts. Camshaft bearings are called sleeve bearings because they are in the shape of a sleeve that fits around the rotating journal or shaft, as shown in Figure 2-29, View A. Connecting rod or crankshaft (main) bearings are of the split or half type, as shown in Figure 2-29, View B. On main bearings, as shown in Figure 2-29, View C, the upper half is installed in the counter bore in the cylinder block. The lower-bearing half is held in place by the bearing cap. On connecting rod bearings, the upper-bearing half is installed in the rod and the lower half is placed in the rod cap. The piston pin bearing in the connecting rod is of the full round or bushing type.
Figure 2-29 – Engine bearings.
The lubrication of bearings is very important to engine service life because it forces oil to high friction points within the engine. Without lubrication between parts, bearings overheat and score from friction.
The journal or shaft must be smaller in diameter than the bearing, so there is clearance (called oil clearance) between the two parts; oil circulates through the clearance. The oil enters through the oil hole and fills the oil groove in the bearing. From there, the rotating journal carries the oil around to all moving parts of the bearing. The oil works its way to the outer edges of the bearing. From there, it is thrown off and drops back into the oil pan. The oil thrown off helps to lubricate other engine parts, such as the cylinder walls, the pistons, and the piston rings.
As the oil moves across the faces of the bearings, it not only lubricates them but also helps keep them cool. The oil is relatively cool as it leaves the oil pan. It picks up heat in its passage through the bearing. This heat is carried down to the oil pan and released to the air passing around the oil pan. The oil also flushes and cleans the bearings. It tends to flush out particles of grit and dirt that may have worked into the bearing. The particles are carried back to the oil pan by the circulating oil. The particles then drop to the bottom of the oil pan or are removed from the oil by the oil screen or filter.
The greater the oil clearance, the faster the oil flows through the bearing; however, excessive oil clearance causes some bearings to fail from oil starvation. Here’s the reason: If oil clearances are excessive, most of the oil passes through the nearest bearings. There is not enough oil for the most distant bearings; these bearings eventually fail from lack of oil. An engine with excessive bearing oil clearance usually has low oil pressure; the oil pump cannot build up normal pressure because of the excessive oil clearance in the bearings.
On the other hand, when the bearings have insufficient oil clearances, there is metal-to- metal contact between the bearings and the journal. Extremely rapid wear and quick failure is the end result. Also, there is not enough throw-off for adequate lubrication of cylinder walls, pistons, and rings.
Engine bearings must operate under tremendous loads, severe temperature variations, abrasive action, and corrosive surroundings. Essential bearing characteristics include the following:
As discussed earlier, there are three basic types of engine bearings—connecting rod bearings, crankshaft main bearings, and camshaft bearings. The backing material (body of the bearing that contacts stationary parts) for engine bearings is normally steel. Softer alloys are bonded over the backing to form the bearing surface. Any one of three basic types of metal alloys can be plated over the top of the steel backing—Babbitt (lead-tin alloy), copper, or aluminum. These three metals may be used in different combinations to design bearings for light-, medium-, or heavy-duty applications. The engine designer selects the combination of ingredients that will best suit the engine.
1. Why is aluminum not used as an engine block?
2. The brackets that hold the engine to the vehicle frame are mounted at a minimum of how many points?
3. The three types of manifold gaskets are the intake, the exhaust and the _.