This is the propeller lock nut for the UMS 5-75cc engine, the five-cylinder 75cc four-stroke radial from UMS.
The part itself is the propeller retaining lock nut.
Why it is replaced: the most safety-critical fastener on the engine, and one that is replaced rather than reused on a critical joint. Supplied as a pack of 4, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Torque to the factory figure, use the specified locking method and check it before every flight.
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