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Spring Retainer (34)

The valve seat in an internal combustion gasoline or diesel engine is the surface against which an intake or an exhaust valve rests during the portion of the engine operating cycle when that valve is closed. The valve seat is a critical component of an engine in that if it is improperly positioned, oriented, or formed during manufacture, valve leakage will occur which will adversely affect the engine compression ratio and therefore the engine efficiency, performance (horsepower), exhaust emissions, and engine life.

Valve seats are often formed by first press-fitting an approximately cylindrical piece of a hardened metal alloy, such as Stellite, into a cast depression in a cylinder head above each eventual valve stem position, and then machining a conical-section surface into the valve seat that will mate with a corresponding conical-section of the corresponding valve.

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.