The valve seat is the area in an internal combustion engine's cylinder head that the intake and exhaust valve actually contact when the valve is in the closed position. Typically made of a special alloy such as Stellite™ or a tungsten-vanadium compound, the valve seat is commonly pressed into the cylinder head and machined to allow for maximum flow and sealing of the intake and exhaust gasses. Both cast iron and aluminum cylinder heads use a valve seat to provide the proper valve closing and sealing of the combustion chamber. Most valve seat machining consists of multiple angles machined on both the seat and the valve head to create both flow and pressure sealing.
Most original equipment engines are designed to run on unleaded or diesel fuel, while the typical performance engine is commonly fueled with unleaded racing gasoline or methanol. Unleaded gasoline does not provide the cushioning effect the leaded fuel provides. This can lead to a valve seat being destroyed by the tremendous forces of the valves being opened and slammed closed. In leaded fuel, the lead is used to cushion this force and provide long-term protection for both the valve and the valve seat. Some engine builders recommend a lead substitute be added to the unleaded fuel to protect the valve train.
The uses of special alloys in the manufacture of the valve seat allow the component to operate on unleaded fuels while resisting the destructive forces caused by the rapidly opening and closing of the valves at high speed. Along with the hammering effect of the valves, extreme heat is also a force to be combated in designing a durable valve train. The harsh environment the valve seat operates in makes alternative cooling by air or oil impossible, so the seat must rely only on the alloy used in its construction to provide operational protection.
When installing a seat into a cylinder head, a hot and cold system of interference fit is often used. By heating the cylinder heads, the machined opening for the seat expands as the material in the head expands. The seats are often placed into a freezer to shrink the molecules in the component and make the seat as small as possible. The frozen valve seat is then pressed into the hot head and allowed to come to an even temperature. The result is a very tightly-fitted seat that will remain in place while being machined, operated and maintained.