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A head gasket is something used in an internal combustion engine to create a seal between the head(s) (containing the valves and (usually) the camshafts) and the engine block (containing the pistons and the crankshaft). The head gasket performs several functions. It aids in creating a close to near-perfect seal in the combustion chamber formed in the space where the head and cylinders meet, so that there is no leakage from the engine in the combustion process. The gasket also acts to seal the interconnecting coolant passages that flow through the head and the block, so that none of the coolant will leak into the combustion chamber or the top of the head where the valves or cams are.

Head gaskets are prone to rupture (or "blowing") due to excessive combustion chamber pressure (i.e. from aftermarket turbocharging systems), or from extreme engine temperature (coolant becoming too hot and boiling, sometimes explosively).

A common modification to some engines is to remove the head gasket and lap the head into the engine block. This is especially popular on small two-stroke engines such as small motorcycles as it can lead to a significant increase in compression and hence more power.

Basically, lapping the head involves removing the gasket and all its remnants, then applying grinding paste to one half or the other, and then rubbing the head against the barrel. This flattens the surfaces against each other, and grinds away the bumps. Finally, the surfaces are cleaned and a very thin layer of gasket goo or even just more grinding paste is sandwiched in the join. This forms a gas-tight seal.

By removing the gasket, the head is closer to the barrel, and thus the unswept area above the piston is made smaller. This increases compression ratio, and higher compression means higher combustion chamber temperatures, which means more power.

Until it blows up, that is.

I should explain this. Increasing the combustion chamber temperature and pressure puts more thermal stress on the head and valves. It also puts more mechanical stress on the seal between the head and the barrel... which you just removed. It's fairly obvious what's going to happen here if you go too far.

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.

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