In some cast-iron cylinder heads, the seats are cut directly into the edge of the valve port. These valve seat areas are machined from the metal of the cylinder head. In some engines, the valve seat area is hardened during manufacture.
In others, hard metal valve seat inserts are pressed into the machined holes. Valve seat inserts are metal rings that match the shape of the valve. They are usually made of an iron alloy. They are used in aluminium cylinder heads to provide a sealing surface for seating the valve.
Leaded fuels leave a deposit on the valve that protects the valve seat. With unleaded gasoline, however this deposit doesn’t occur, and all cast iron heads used with unleaded gasoline have hardened valve seats.
The faces of the valve are ground at an angle of 45 degrees or 30 degrees. Some engines use 30 degrees or 45 degrees face angles for inlet valves, and 45 degrees for exhaust valves.
Valve seats are often ground to the same angle as the valve face, but they can differ. The difference is called an interference angle. An interference angle allows for a quick bedding-in of the valve face to the seat on new engines. It may also allow for slight changes in angle as a valve heats and expands.
