MAJOR COMPONENTS & FUNCTIONS
* Engine Block — Holds the cylinders and crankshaft. On “pushrod” engines, the block also holds the camshaft. On “overhead cam” (OHC) engines, the camshaft(s) are in the cylinder head(s). Engine blocks come in various configurations including in-line three, four, five and six cylinders, V-blocks (V6, V8 & V10), and horizontally opposed (Porsche, Subaru and older air-cooled VW). Usually made of cast iron, some blocks may also be aluminum with or without iron cylinder liners. Cylinder bores experience wear and may have to be bored or honed to oversize when the engine is rebuilt.

* Cylinder Heads — Mounted atop the cylinders, these hold the combustion chambers, valves, upper valvetrain components and spark plugs. Heads may be cast iron or aluminum. Cast iron heads usually have integral valve guides and seats, but some have replaceable guides (big block Chevy V8s). All aluminum heads have replaceable guides and seats. Heads may crack or warp if the engine overheats. The head may need to be resurfaced if the head gasket has failed.

* 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 length of the throws determines the “stroke” of the crankshaft. Bearing journals can become scored, worn or misshapen at high mileage, requiring the journals to be reground to undersize before new bearings are installed. Crankshaft kits include a reground crankshaft and new undersized bearings.

* Pistons — The pistons are the reciprocating components in the cylinders that compress the air/fuel mixture and transmit the force of combustion to the connecting rods. Pistons may be cast aluminum, a special high-strength “hypereutectic” alloy, or forged aluminum. Oversized pistons and rings are required if the cylinders have been bored and honed to a larger size to eliminate wear. Pistons must be replaced if scuffed, burned, cracked or worn. Some replacement pistons have anti-friction coatings on the sides to prevent scuffing.

* Piston Rings — Most pistons have three rings: a top compression ring, a middle or 2nd compression ring and a lower oil ring. Rings come in various thicknesses and materials. Least expensive are plain cast iron rings. Rings may be chrome plated or faced with moly to reduce wear. Top compression rings in higher output engines are often ductile iron or steel. Worn or broken rings reduce compression, increase blowby into the crankcase (which contaminates the oil) and increases oil consumption.

* Connecting Rods -Attach the pistons to the crankshaft. May have to be replaced if twisted or cracked. A “wrist pin” at the top attaches the small end to the piston. Rod failures can be caused by overrevving the engine, or loss of lubrication (spun bearing).

* Bearings — Support the crankshaft and camshaft on a thin film of oil. Bearings may be aluminum or “tri-metal” with a thin layer of soft babbit over copper/lead on a steel shell. Worn bearings can produce noise and low oil pressure. Bearing failure can damage the crankshaft and connecting rods, and can be caused by a loss of oil pressure or not changing the oil often enough.

* Oil pump — Pumps oil from the oil pan to lubricate the engine. May be mounted in the crankcase and driven off the distributor, or mounted inside the front timing cover and driven off the crankshaft. Worn pump gears can result in low oil pressure.

* Camshaft and Lifters/followers — These components actuate the valves. The camshaft has lobes on which the lifters or followers ride to open the valves. The profile and position of the lobes determines the power and torque characteristics of the engine. Most newer pushrod engines use roller lifters to reduce friction. Older pushrod engines use flat bottom lifters. New lifters should be used with a new camshaft.

* Timing components (gears, chains and belts) — These are used to drive the camshaft (which turns at half the speed of the crankshaft). Timing components should be replaced if worn or if the chain has stretched. Most rubber timing belts should be replaced at 60,000 or 100,000 mile intervals to reduce the risk of breakage. “Interference” engines will allow the valves to hit the pistons if a timing belt breaks.

* 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, crack or leak compression at high mileage. Exhaust valves are often replaced when doing a “valve job.” New springs may also be required if the old springs are weak. Valve guides may be “integral” (part of the cylinder head itself), or a cast iron or bronze insert. The guide supports and positions the valve stems. Worn valve guides or valve guide seals are a common cause of oil burning in high-mileage engines. Worn valve guides can be reconditioned a variety of ways, including sleeving them with bronze or cast iron liners, replacing them with new guides, or reaming out the old guide and installing new valves with oversized stems.

RELATED ITEMS
* Motor oil — Keeps the engine lubricated, helps cool the bearings and pistons, and allows hydraulic lifters and followers to maintain proper valve lash. Oil comes in various viscosities (which is the thickness of the oil). Most popular are multi-viscosity oils such as 5W-30, 10W-30 and 10W-40. Many new vehicles require 5W-20 or 0W-20. Heavier oils such as 15W-40 and 20W-50 may be used in performance or hard-working engines during hot weather. The first number refers to the cold oil thickness, and the second number to its thickness when it is hot.
Oil becomes contaminated with combustion blowby products, moisture and wear particles, and should be changed regularly for preventive maintenance. Failure to change the oil often enough can lead to oil breakdown, sludge and engine failure.
* Gaskets — See ASE P2 Test Section on Gaskets.
* Fasteners (head bolts, rod bolts, pan & cover bolts) — Many late model engines use “Torque-To-Yield” (TTY) head bolts that stretch slightly when tightened. TTY bolts should not be reused if the head gasket is replaced, otherwise they may break.
* Assembly lube — Recommended for assembling engine parts.
* Motor mounts — Dampen engine vibrations. Should be replaced if collapsed or broken.
* Special tools — These include a torque wrench, angle gauge, ring compressor, Plastigage, cylinder deglazer, harmonic balancer puller or other tools that may be needed to perform engine repairs.

INSTALLING ENGINE PARTS
Engine kits provide most of the parts normally required when rebuilding an engine (bearings, pistons, rings, gaskets, etc.). Engine rebuilding requires precision tolerances and a high level of skill. Machining services such as cylinder boring/honing, crankshaft regrinding, valve seat resurfacing, cylinder head resurfacing, crack detection and cleaning are usually required.
  
Piston rings must be installed with a certain side up, and a ring expander should be used to minimize the risk of ring damage or breakage.
 
Bearings clearances are critical and must be checked to make sure they are not too tight or too loose.
 
Head bolts must be tightened in a specified sequence to specifications with a torque wrench and/or angle gauge (if required). Rod and main bolt torque is also critical.