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ASE PS2 Test Preparation Guide: Engine Parts


10/10/2011
By Larry Carley

 

● BEARINGS — Provide support for the crankshaft and camshaft. A thin film of oil between the bearings and shafts reduce friction and prevent wear. Most late model original equipment engine bearings are aluminum, or aluminum with a steel backing. Older engines typically use tri-metal (babbit and copper/lead over steel) rod and main bearings. 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 (undersized bearings are required for reground crankshafts).

● OIL PUMP — Pumps oil from the oil pan to the rest of the engine to lubricate the crankshaft bearings, camshaft and valvetrain components. The pump may be located in the crankcase or the front engine cover. 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 old parts.

● OIL FILTER — Keeps the oil clean by trapping dirt and wear particles that can cause engine damage and wear. Replacement is recommended when changing the oil (typically every 3,000 to 5,000 miles). Replacement filter must be the correct one 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.

● PISTONS — The reciprocating components in the cylinders that compress the air/fuel mixture and transmit the force of combustion to the connecting rods and crankshaft. 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. Ring grooves on some pistons may be hard anodized for wear resistance.

Pistons can be damaged or cracked by detonation (spark knock) and burned by pre-ignition (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 also is 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.

● PISTON RINGS — Seal the piston against the cylinder to hold cylinder pressure and compression. Most pistons have 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. Follow the ring supplier’s recommendations.

Rings are directional and must be installed on pistons right side up (the top is marked with a dot). The lateral thickness of the rings also must match the depth of the ring grooves in the pistons. 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. 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.

● CRANKSHAFT — 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. Critical dimensions are main and rod journal diameters. New crankshaft seals should be installed when 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. Most late model engines have roller lifters (pushrod engines), or roller followers (OHC engines) if the cams are mounted in the cylinder head. Older pushrod engines usually have flat bottom lifters (tappets). Engines with flat tappet cams may require a ZDDP oil additive to prevent lifter and cam wear.

The lifters move the 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. The cam 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. Some late model OHC engines use variable valve timing (VVT) to improve performance, fuel economy and emissions. VVT uses a mechanism on the cam drive sprocket to advance or retard cam timing.

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 (sometimes both). The camshaft(s) rotates at half the speed of the crankshaft, and also may 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 to 100,000 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 — Includes valves, valve springs, valve retainers, valve guides, valve guide seals, rocker arms, 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 also may 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, relined or replaced. Replacement guides are either bronze or cast iron. Bronze liners can be 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 & SEALS — Engines contain a variety of gaskets to seal fluids and gases. Head gaskets, which may have non-asbestos or graphite facings on a solid or perforated steel substrate, seal the cylinder head to the block. Some 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 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. Most mounts are solid rubber, but some vehicles use “hydroelastic” (fluid filled) mounts or electronic mounts. A broken motor mount typically causes drivetrain noise when accelerating.

 

Cooling System

Electrical System

Exhaust Parts

Ignition System

Manual Transmission/Transaxle Parts


Suspension & Steering Parts

Fuel System

Engine Parts

Emission Controls

Driveline Components


Heating, Ventilation & Air Conditioning (HVAC)

Brakes

Automatic Transmission/Transaxle
 















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