A head gasket is a gasket used to form a seal between the head and engine block of most internal combustion engines. A very small number of engines do not use a gasket to seal the head, some use a single metal ring (usually aluminum to seal each cylinder.

The vast majority of modern engines use a "flame ring" gasket design. This consists of a high temperature fibrous sheet, with holes cut to fit the cylinder bores, coolant galleries and lubrication feed and return passages. Flame ring gaskets require the use of gasket sealant.

Older engines often use copper head gaskets, These require a somewhat more accurately machined set of mating surfaces. These gaskets are designed to be used with no sealants, however various manufacturers make sealants designed to improve the reliability of this type of gasket.

Many manufacturers require the bolts which secure the head to the block to be tightened to specification torque settings at regular intervals (typically every 20-50,000 miles). Failing to observe this maintenance is the most common cause of head gasket failure. When either tightening in this maintenance, or in removing or installing a head a proper sequence must be followed. Generally tightening proceeds from near the center of the head and alternating to opposite sides, loosening is usually the reverse order of the tightening sequence. Failure to use the correct order (RTFM), will usually result in a warped head, requiring machining the surface true.

In the event that either the head or block needs to be resurfaced, the dimensional changes introduced may requre the use of a thicker head gasket to maintain the correct compression ratio. These are available from the engine maker.

How to diagnose head gasket failures

Generally, when a head gasket fails, coolant or lubricating oil may intermix or either of these fluids may enter the cylinders.

The most common failure in a head gasket is at the cylinder bores. Usually this causes coolant or oil to be drawn into the combustion chamber. Look for either the smell of coolant (both ethylene- and propylene- glycol have a 'sweet' odor} in the vehicle's exhaust or for the smell of exhaust in the coolant.

Coolant entering into the cylinders will quickly remove the normal formation of carbon soot from the interior of the cylinder wall. Visual inspection of the spark plugs or the top of the piston will quickly identify this problem.

When oil enters the cylinders, it will often cause the spark plugs to foul which is recognized by a carbon deposit on the plug's electrodes and insulator. Carbon on the insulators can also cause a plug to not spark by creating a conductive film on top of the insulator, resulting in a lower resistance electrical circuit.

The presence of oil in the engine's coolant usually is the result of a failed head gasket. The oil sump below the engine, however normally contains some water due to condensation of the the fraction of combustion which blows past the piston rings. Coolant can be recognized by its color.