High hardness approx. 55HRC steel, circular parts for shims approx. With 0.05mm size steps. These can be set, especially in machine shop and service conditions, to set the optimum valve clearance, which can be measured between the valve end and the clearance insert when the valve is closed. It is important to close the valve completely to dissipate the heat of combustion produced through the valve seat and to avoid pressure loss. Proper "chipping" is important to ensure that the valves close properly over a long period of time.