Valve shim
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A valve shim is a disc of hardened metal of precisely calibrated height used to adjust the clearance (US:lash) between a 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. In situations where valve-train weight reduction is important top-hat shims are used as an alternative.