Bearings provide support for the crankshaft and camshaft. A thin film of oil between the bearings and shafts reduces friction and prevents wear. Bearings usually have a steel backing with an aluminum, copper/ lead and/or babbitt overlay. Worn bearings can produce noise and low oil pressure. Bearing failure can occur if a bearing overheats and seizes, cracks or pounds out. Bearings can be damaged by dirt, lack of lubrication or oil breakdown. Wear can be accelerated by neglecting regular oil and filter changes. Bearings are usually replaced as complete sets, and must be the correct size for the shaft journals. If the crankshaft has been reground to undersize, undersize bearings are required to maintain proper oil clearances.
The oil pump pumps oil from the oil pan to the rest of the engine to lubricate the crankshaft bearings, camshaft and valvetrain components. Worn pump gears can result in low oil pressure. Recommend a new oil pump if the bearings are being replaced or the engine is being overhauled. A new oil pickup tube and screen should also be installed because it is difficult to thoroughly clean the old parts.
The oil filter traps dirt and wear particles that can cause engine damage and wear. They should be replaced at every oil change (typically every 3,000 miles). They must also be the correct filter for the application, as some side-mounted filters contain check valves to prevent oil from draining back into the engine after the engine is shut off.
The reciprocating components in the cylinders that compress the air/fuel mixture and transmit the force of combustion to the connecting rods and crankshaft are the pistons. Pistons may be cast aluminum, a special high-strength "hypereutectic" alloy or forged aluminum. The latter two are used in performance applications. Some pistons have moly coated skirts to prevent scuffing.
Pistons can be damaged or cracked by detonation (spark knock) and burned by preignition (hot spots in the combustion chamber) or an overly lean fuel mixture. Wear in the cylinder increases clearances and can cause "piston slap" when the engine is cold.
Replacement pistons must be the correct size for the engine. If the cylinders have been overbored to correct wear, oversized pistons are required. Compression ratio is also important and can vary according to piston design, bore diameter and type of cylinder heads on the engine. Piston installation requires a ring compressor tool, and may require press fitting the wrist pins unless the pistons have full-floating wrist pins with locks at each end.
Three rings are used to seal the pistons: a top compression ring, a middle compression ring and a lower oil ring. Worn or broken rings reduce compression, increase blowby into the crankcase (which contaminates the oil) and increases oil consumption. Rings may be cast iron, ductile iron or steel, and plain, chrome plated, nitrited or moly faced. Most late-model engines use moly faced rings. Some high-output engines require ductile iron or steel-top compression rings for durability. Replacement rings should be the same grade of material or better than the original. Rings are directional and must be installed on pistons a certain way. A ring expander tool should also be used to minimize the risk of ring damage or breakage during installation.
Connecting rods attach the pistons to the crankshaft and rods must be replaced if bent, twisted or cracked. A "wrist pin" at the top attaches the small end to the piston. For performance applications, forged steel and lightweight aluminum and titanium rods are available. Critical dimensions are overall rod length and bore diameters.
The crankshaft is a rotating cast iron or forged steel shaft with throws and counterweights that transmits the up and down reciprocating motion of the pistons into torque. The bearing journals on the crankshaft can become worn or misshapen at high mileage, requiring the journals to be reground to undersize when the bearings are replaced. Crankshaft kits include a reground crankshaft and new undersized bearings. The critical dimensions are the main and rod journal diameters. New crankshaft seals should be installed when the crankshaft is replaced.
CAMSHAFT AND LIFTERS
The camshaft opens and closes the valves. In pushrod engines, the camshaft is located in the engine block. Hydraulic lifters ride on the cam lobes. Lifters may be flat bottomed or have wheels (roller lifters). The lifters push up on pushrods that operate the rocker arms atop the cylinder head to open the valves. In overhead cam (OHC) engines, the camshaft is located in the cylinder head. It may sit atop the valves and open the valves directly, or it may be offset and use followers to open the valves. OHC engines may use separate cams for the intake and exhaust valves, and have as many as four cams for a V6 or V8 engine.
The profile and position of the lobes determines the power and torque characteristics of the engine. High-lift/long-duration cams provide more high-speed power. Various types of aftermarket replacement cams are available to improve engine performance, torque and fuel economy. New lifters should be used with a new camshaft. A high-pressure assembly lube should be used on the cam lobes along with a special break-in procedure to assure proper lubrication during break-in.
TIMING GEARS, CHAINS AND BELTS
The camshaft is driven by gears connected to the crankshaft, or by a timing chain or belt. Pushrod engines typically use gears or chains, while overhead cam engines use belts or chains (and sometimes both). The camshaft(s) rotates at half the speed of the crankshaft and may also drive the distributor or a mechanical fuel pump on older vehicles. Timing components should be replaced if worn or if the chain has stretched. Timing chain sets include a new chain, camshaft gear and crankshaft gear. Most passenger car engines use a "silent" chain design, but performance engines often use a stronger "roller" chain. Most rubber timing belts should be replaced at 60,000-mile intervals to reduce the risk of breakage. Some engines (called "interference engines") do not have enough clearance between the valves and pistons to prevent contact and damage if a timing belt fails.
UPPER VALVETRAIN COMPONENTS
The upper valvetrain components includes valves, valve springs, valve retainers, valve guides, valve guide seals, rocker arms and pushrods. Exhaust valves run much hotter than intake valves and may burn and leak compression at high mileage. Exhaust valves are often replaced when doing valve work. New springs may also be required if the old springs are weak. The valve guides support the valve stems. The guides are often worn at high mileage causing an increase in oil consumption. Worn guides must be reconditioned or replaced. Replacement guides are either bronze or cast iron. Bronze liners are often installed in integral guides to restore the inside diameter. At the top of the guides are seals to control lubrication. "Umbrella" seals act like a shield to deflect oil away from the guides, while "positive" seals fit tightly around the stem to control lubrication. Recommend new valve springs if a customer is replacing a camshaft.
GASKETS AND SEALS
Engines contain a variety of gaskets to seal fluids and gases. Head gaskets, which may have nonasbestos or graphite facings on a solid or perforated steel substrate, seal the cylinder head to the block. Some are have a multi-layer steel (MLS) construction. A head gasket failure can be caused by overheating, resulting in internal coolant leaks and/or a loss of compression. Replacing a head gasket may require resurfacing the cylinder head.
Valve cover and oil pan gaskets may be cork or molded silicone rubber. Some engines use RTV silicone instead of a gasket to seal covers (replacement gaskets are usually available for such applications if a customer prefers using a gasket). Other gaskets include timing cover, manifold and thermostat as well as crankshaft and valve seals. These may have to be replaced when doing internal engine work, or to fix an oil, coolant or vacuum leak.
Sealers should not be used on most head gaskets, but they may help seal pan and cover gaskets.
Gaskets are often sold in sets that provide all the gaskets and seals needed to complete a certain type of repair.
Motor mounts attach the engine to the chassis and help cushion engine vibrations. There are two types: solid rubber and "hydroelastic" (fluid filled). A broken motor mount typically causes drivetrain noise when accelerating.