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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 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 preferred, because they require no adjustment, they do not work well in engines capable of high RPMs. In those, shims are most often used. As an example, most motorcycle engines capable of 8000RPM+ use shim-adjustable valves.
A top-hat shim (US: lash cap) is a small, inverted, hardened-metal cup, placed atop a valve stem in an internal combustion engine. Varying thicknesses of top-hat shims are used to adjust valve clearances in much the same way as standard flat disc shims (as used in Fiat SOHC/DOHC, Volvo, and early Jaguar engines), where the cam-lobe operates directly on the valve without an intermediate rocker arm. Top-hat shims offer a dramatic weight-reduction over standard disc shims, and are used where valve-train weight reduction is important (i.e. high-performance road- and race-orientated engines).
In competition engines, where aggressive cam action is common, top-hat shims are used to protect the tip of the valve stem. For this reason[clarification needed] they are necessary with titanium valves.
Top-hat shims are also often used to attain a required valve-stem protrusion.
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