Compressed gasket thickness is critical in an engine’s specification. By juggling the compressed gasket thickness, and physically changing the piston deck clearance figure, the compression ratio changes in an engine can prove startling. Don’t take these numbers for granted. If you do, you could be sorry. See the text for more info (and examples)
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Why Are Head Gaskets So Important?
According to a number of manufacturers, the gaskets between the cylinder head and the block form the most critical seal on an engine. In a stock, high output production engine, combustion pressures up to 1,000 PSI are seen on normally aspirated gasoline power plants while combustion temperatures will sometimes exceed 2,000° F. In a race application, combustion pressures are much higher. According to Fel Pro, a GM-sponsored Winston Cup engine test saw combustion pressures in excess of 2,500 PSI! Obviously, the head gasket has to seal these forces.
One of the most important requirements of building an engine is to keep everything as clean as possible; this applies to gaskets and gasket mating surfaces also. Every surface a gasket comes in contact with must be free of dirt, water, oil, grease, etc. The surface must also be flat -- warpage should not exceed .0025-.0030-inch in any direction. Take special care to clean surface of old gasket material that may have stuck to either the part (oil pan, intake manifold, etc.) or the cylinder heads and block. One point that is often overlooked is keeping the gaskets themselves clean.
But there's more: The head gasket must also seal coolant and hot (thin) oil -- both flowing under pressure between the cylinder block and the cylinder head. According to Fel Pro, this job is compounded because modern coolant formulas as well as oil detergents and additives tend to cling to surfaces and soak into gaskets. Gasket materials must be chosen carefully to resist these fluids and to maintain an effective seal.
Fel Pro also points out that head gaskets must resist forces that tend to scuff gasket surfaces and inhibit proper sealing. Engine vibration and head shifting and flexing all result from combustion pressures. And in today's race engines (which are often combinations of aluminum heads and iron blocks), material expansion rates are much different. Aluminum expands about twice as much as cast iron. Because of this uneven expansion, a shearing action is created which can wreak havoc on the head gasket.
Finally, head gaskets must resist crushing from cylinder clamping forces (i.e.: the head bolts and/or studs). Believe it or not, these clamping forces can exceed 200,000 pounds in some cases. The head gasket must accommodate all of these situations and do so on a regular basis.