This is the rocker arm screw for the UMS 5-125cc engine, the five-cylinder 125cc four-stroke radial from UMS.
The part itself is the rocker arm adjuster screw, which sets valve clearance.
Why it is replaced: a scored or rounded adjuster makes it impossible to hold a setting, and these are replaced at overhaul in the usual course. Supplied as a pack of 10, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Back the screw off before adding feeler gauges, then lock the nut without moving the setting.
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