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Head gaskets seal the combustion chamber between the engine block and cylinder head. They can be single-use or multi-use and may use graphite or silicone sealant. Proper machining of surfaces is crucial, and special gaskets are needed for contrasting materials to prevent tearing.
A head gasket is used to seal the combustion chamber by filling the space between the engine block and the cylinder head. Consisting of compounds surrounding a thin layer of metal, the head gasket flattens to size when the stock is tightened into place. Typically a single-use type of component, some specialty solid copper or aluminum head gaskets fabricated from material in dead soft hardness are capable of multi-use applications. Used without any other sealing compound, this type of high performance head gasket is capable of effectively sealing the combustion chamber and water passages against any leakage.
Some types of head gaskets use a graphite compound on the outer surface of the gasket. This graphite material acts as a sealant and ensures that no leaks develop over time. As the engine reaches normal operating temperature, the graphite bonds with the head and locks the surfaces to create a permanent bond and seal. When stripped, this graphite is often very difficult to remove and usually requires a razor blade or sturdy putty knife to scrape it off. Most engine manufacturers recommend applying a small amount of silicone sealant to the corners of the head gasket as well as around the coolant ports to provide an added measure of protection against leaks.
One of the most important factors in creating a proper seal between engine components and the head gasket lies in the proper machining of the cylinder head and engine block surfaces. Creating a smooth, level surface is not enough when machining parts. Providing the correct surface to dig into the joint is a matter of great importance. If the surface is too smooth, the joint will have nothing to bite on; If it is too rough, the gasket will not be able to fill the cracks. The machinist must know what type of head gasket will be used to ensure that he puts the proper surface on the parts as they are machined.
When contrasting materials are combined, 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. If the proper gasket is not used, the gasket will tear as the two sides stretch at different rates as the engine heats up. Many head gasket problems stem from failure to use the proper type of gasket.
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