Main role of valve seats is to provide impermeability with the valves. It controls exit of exhaust gases for exhaust valves and enter of fuel to cumbustion chamber.

   It is mounted to head cover. There are also some seats in the formed to cover own surface. During the engine overhouls, necessary attention should be paid to the correct alignment of valve, valve guide and seat insert ring. Misalignments may cause alternating bending stresses on the valve stem, valve seat, valve tip and valve guide surfaces. Engine power decreses, some wearings, cracks, burnings and finally failures may take place. In addition position tolarances like roundness, run-out, roughness are also important for confor nable running. Seat ring should be mounted to cover in suitable clearance according to standards. These factors affetc the performance of valve directly. It needs careful attention during the machining of seat surface.

   In the long run, seat rings may wear and fall down and these used rings may be changed with new ones. however, this requires high attention so not prefferred on the contrary thick valve is used.

   Seats provide wear resistance to high temperature as material. Materials of them are high Cr alloy and hardness materials as steel or white cast iron.