A top-hat shim (lash cap US) 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 a similar fashion to 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 in applications 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, lash caps are used to protect the tip of the valve stem. For this reason they are a necessity when titanium valves are used.
As well, lash caps are often used to attain a required valve stem protrusion.
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