Valve shim

A valve shim is a disc of hardened metal of precisely calibrated height used to adjust the clearance (US:lash) between the back side of the cam-lobe and the valve on which it operates where the cam-lobe operates directly on the valve without an intermediate rocker arm. The shim sits atop a shim bucket which itself sits atop the valve-stem and moves up and down within a machined bore. While hydraulically self adjusting valves are sometimes used for their lack of required adjustment they do not work well in engines capable of high RPMs. In situations like these shims are most often used. As an example most motorcycle engines capable of 8000RPM+ use shim adjustable valves.