This is the spring washer inner for the UMS 7-77cc engine, the seven-cylinder 77cc four-stroke radial from UMS.
The part itself is the inner spring washer, which keeps a fastener from loosening under vibration.
Why it is replaced: vibration on a radial will undo a fastener without a locking device, so these are renewed at service. 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-flatten a washer, which will not hold tension again.
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