Head gasket

A head gasket is a gasket that sits between the engine block and cylinder head(s) in an internal combustion engine.

Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in any engine, and, as part of the combustion chamber, it shares the same strength requirements as other combustion chamber components.

The cost of a replacement gasket is usually not extreme, but the price of total repair is significantly high. This is because the process of removing/replacing an engine head is very time consuming - around 75% of cost will be labor. Furthermore, untreated blown gaskets usually seriously damage the engine, then requiring even more expensive work.

The usual internal combustion engine is a water-cooled four-stroke gasoline design. It uses three circuits of working fluids:

These are all vital, but must never be allowed to intermix. Apart from sealing the cylinder, the head gasket also seals water and oil conduits between the head and block. Any connectivity between them will cause engine failure, or significant problems like burning oil (smoke from the exhaust). Blue exhaust smoke may be indicative of oil burning. White exhaust smoke is an indicator of coolant burning, another sign of head gasket wear.

Occasionally, the compression in the cylinder will cause a leak to form in the gasket and the gasket will have to be replaced, or severe damage can take place (a "blown" head gasket). This problem has been exacerbated by the use of aluminum rather than iron cylinder heads; while lighter than iron, aluminum has a much greater thermal expansion rate, which in turn causes a great deal more stress to be placed on the head gasket. Engine makers have responded to this by adding a non-stick coating such as Teflon to the surface of the head gasket.

If the gasket fails, a variety of problems can occur, from compression loss (leading to power reduction, or a rough engine), to exhaust gases being forced into the cooling system, leading to the engine overheating and increased engine wear due to the motor oil being mixed with antifreeze. Coolant can leak into the cylinders, causing the exhaust to issue steam and the catalytic converter to be damaged. If a very large amount of coolant does this, hydrolock can occur, causing extensive engine damage. Sometimes, all that may happen when a head gasket is blown is excessive steam erupting from the tailpipe; yet the engine may act and drive like normal, until all the coolant is gone and the engine overheats. In other occurrences compression leaks into small spaces between the gasket, and either the cylinder head or engine block traps air that is then released when the engine is turned off. The air then escapes into the coolant and creates air pockets. Sometimes these air pockets can get trapped in the engine's coolant thermostat, causing it to stay closed and cause further overheating, thereby creating still more voids between the gasket and the engine. Air pockets can also cause the engine to expel coolant into the overflow or expansion tank, thereby reducing the amount of coolant the engine has available to stay cool.

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Wikipedia