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Different kinds of gaskets, #4 indicating a V4 head gasket

A head gasket is a gasket between the engine block and cylinder head 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.

• MLS - stands for "Multiple Layers Steel,". Most modern head engines are produced with MLS gaskets. These typically consist of three layers of steel. The contact faces are usually coated with a rubber-like coating such as Viton that adheres to the cylinder block and cylinder head respectively whilst the thicker center layer is bare.

• Solid Copper - A solid sheet of copper, and typically requires special machining called 'o-ringing' that places a piece of wire around the circumference of the cylinder to bite into the copper. When this is performed copper gaskets are extremely durable. Recently companies have started producing copper gaskets with integral sealing wires permitting their retrofit into engines without the removal of the engine block for machine work.

• Composite - Older technology. Typically these are made from asbestos or graphite but are more prone to blowouts than newer gaskets. Asbestos gaskets are becoming increasingly rare due to health concerns.

Occasionally, the compression in the cylinder may cause a leak to form in the gasket (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 blows, 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). If a very large amount of coolant leaks into the cylinders, 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 and the engine may act and drive like normal.

Head gasket condition can be investigated by checking the compression pressure with a pressure gauge, and the pressure in the cooling system. Driving with a blown head gasket (if possible) can cause additional extensive damage.