A head gasket is used to seal the combustion chamber by filling the gap between the engine block and the cylinder head. Consisting of composites surrounding a thin metal layer, the head gasket is flattened to size when the cylinder head is torqued into place. Typically a one-time use type of component, some special solid copper or aluminum head gaskets crafted from material in dead soft hardness are capable of multi-use applications. Used without any further sealing compound, this type of high-performance head gasket is capable of effectively sealing the combustion chamber as well as the water passages against any leaks.
Some head gasket types utilize a graphite compound on the outer surface of the gasket. This graphite material acts as a sealant and ensures no leaks develop over time. As the engine comes up to normal operating temperature, the graphite bonds with the head and block surfaces to create a permanent bond and seal. When disassembled, this graphite is often very difficult to remove and typically requires a razor blade or a sturdy putty knife to scrape it clean. Most engine manufacturers recommend a small dab of silicone sealant be applied to the corners of the head gasket as well as around the coolant ports to provide an extra measure of protection against leakage.
One of the most important factors in creating a proper seal between the engine components and the head gasket lies in the proper machining of the cylinder head and engine block surfaces. Creating a smooth and level surface is not enough when machining the pieces. Providing the correct surface to dig into the gasket is a matter of great importance. If the surface is too smooth, the gasket will not have anything to bite into; if it's too rough, the gasket will not be able to fill the crevices. The machinist must know which type of head gasket will be used to ensure he puts the proper surface on the parts as they are machined.
When combining contrasting materials, such as using aluminum heads on a cast iron block, special head gaskets must be used to effectively seal the engine. This is due to the different expansion rates of the two different metals. Failure to use the proper gasket will result in the gasket being torn apart, as the two sides are stretched at different rates as the engine heats up. Many head gasket issues come from the failure to use the proper type of gasket.