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A valve head, or cylinder head as it is more commonly called, is a piece of an internal combustion engine that contains and houses the intake and exhaust valves. The head sits on top of an engine directly over the pistons and encloses the combustion chamber. The valve head has an opening known as a valve guide running completely through the valve head.
The valves are located in the bottom of the valve head facing the piston and are held in place by having their stems running up through a valve guide seal. This is located inside of the valve spring. A valve retainer and keeper clip hold the entire component in place.
One of the major concerns when designing a valve head is to plan a cooling system that will prevent overheating. Overheating of the valve head leads to head gasket failure and can even lead to cracking of the head. Water passages are cast into the head as it is manufactured, and they allow coolant to flow through the heads and keep them cool. The water passages align with water passages cast into the engine block. The coolant flows freely between the block and heads and absorbs the heat from the combustion process.
A valve head can be made of cast iron or aluminum. Most modern heads are of aluminum construction due mainly to the lighter weight of the material as compared to iron. Aluminum heads also produce more power due to their ability to absorb the engine heat differently and more effectively than their iron counterparts. The expansion rates of aluminum and cast iron are very different; special head gaskets must be used to allow the two materials to expand and constrict at different rates without damaging the gasket or its seal.
One of the best methods of producing horsepower from an engine is to perform extensive work in the valve pockets and exhaust areas of a valve head. By grinding and smoothing the areas where the intake charge and exhaust flow into and out of the head, the power levels of the engine can be drastically raised. This type of work is known as porting. Even the basic and simple act of matching the intake and exhaust ports to the openings in the corresponding gaskets will gain horsepower. There is a very fine line between improving the flow of a head and ruining the head, which is why this type of work is very expensive to have done by a professional.