This is the o-ring (top cover) for the UMS 5-210cc engine, the five-cylinder 210cc four-stroke radial from UMS.
The part itself is the O-ring for the top cover joint.
Why it is replaced: a single-use sealing ring renewed whenever the cover is disturbed. Supplied as a pack of 10, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Lubricate lightly with the correct oil and seat it fully.
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