These tappets are used between the cam lobes and the valves. The case hardened steel housing holds a removable steel wafer that pushes down on the valve stem and spring to open the valve. Changing the thickness of the wafer adjusts cam lash and sets the valve clearance. Valve lengths are never precisely equal, but even a difference of 0.001 inch is a problem at the cam. Using various puck thicknesses takes up those differences.