Valve seats in internal combustion engines are made of special alloys like Stellite or tungsten-vanadium composite to provide maximum flow and sealing of intake and exhaust gases. Unleaded fuel can damage valve seats, but special alloys resist these forces. Hot and cold interference fit systems are used to install valve seats.
The valve seat is the area in the cylinder head of an internal combustion engine that the intake and exhaust valves actually come into contact with when the valve is in the closed position. Typically made from a special alloy such as Stellite or a tungsten-vanadium composite, the valve seat is commonly pressed into the cylinder head and machined to allow for maximum flow and sealing of intake and exhaust gases. Both cast iron and aluminum heads use a valve seat to provide proper shutoff and sealing of the combustion chamber. Most valve seat machining consists of multiple angles machined into both the valve seat and head to create a pressure and flow seal.
Most original equipment engines are designed to run on unleaded or diesel fuel, while the typical performance engine is fueled by unleaded racing gasoline or methanol. Unleaded gasoline does not provide the damping effect that leaded fuel does. This can lead to a valve seat being destroyed by the tremendous forces of the valves being slammed open and closed. In leaded fuel, lead is used to dampen this force and provide long-term protection for both the valve and valve seat. Some engine manufacturers recommend adding a lead substitute to unleaded fuel to protect the valve train.
The uses of special alloys in the manufacture of the valve seat allow the component to run on unleaded fuels while resisting the destructive forces caused by the rapid opening and closing of valves at high speed. Along with the hammering effect of the valves, extreme heat is also a force to be reckoned with in designing a durable valvetrain. The harsh environment in which the valve seat operates makes alternate air or oil cooling impossible, so the seat must rely solely on the alloy used in its construction to provide operational protection.
When installing a seat in a stock, a hot and cold interference fit system is often used. When heating the stocks, the machined opening for the seat expands as the material in the stock expands. Seats are often placed in a freezer to reduce the molecules in the component and make the seat as small as possible. The frozen valve seat is pressed against the hot head and allowed to come to a uniform temperature. The result is a tight fitting seat that will stay in place while being machined, operated and maintained.
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