This is the rocker arm nut for the UMS 5-180cc engine, the five-cylinder 180cc four-stroke radial from UMS.
The part itself is the lock nut that holds the rocker adjuster setting.
Why it is replaced: a nut that no longer locks lets the valve clearance drift and is a common cause of a noisy top end that reappears after adjustment. Supplied as a pack of 5, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Replace rather than re-tighten a nut that has lost its friction.
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