This is the o-ring (bottom cover) for the UMS 5-100cc engine, the five-cylinder 100cc four-stroke radial from UMS.
The part itself is the O-ring for the bottom cover joint.
Why it is replaced: renewed on every removal to stop the oil weep that otherwise reappears immediately. Supplied as a pack of 10, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Check for a distorted cover face if the new ring still leaks.
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