A head gasket is a piece of material which is located between the head and the block inside an engine. It is usually made of a material which is softer than the head and block to allow for sealing of imperfections between the two mating surfaces. It also seals ports which transfer oil or coolant between the two parts.
Here is a picture of a typical head gasket:
The head gasket has several jobs to perform:
- Seals the combustion chamber
- Keeps the oil in the oil and the water in the water, and both out of the combustion chamber
- Allows for movement of the head/block surfaces to ensure proper sealing while the engine is running. This is really important if the engine block is made of one material (iron) and the head is made of another (aluminum), as the two metals expand and contract at different rates during the heating/cooling cycles.
As stated, the head gasket can be made of many different materials. There are several different types:
- Multi-Layer Steel (MLS) - As the name implies, it's made of multiple layers of steel. Since the layers are thin, they will deform under the tight load of head as it's clamped to the block.
- Pure Copper - Usually has a ring embedded within it at the cylinders to enclose the cylinder pressures.
- Composite - Older tech. Usually made of graphite and/or asbestos.
- Elastomeric – utilized a steel core plate with molded in place silicone rubber beads to seal oil and coolant passages. The bores were sealed by rolled steel fire rings in a more conventional manner.