This is the wood ruff key for the UMS 7-77cc engine, the seven-cylinder 77cc four-stroke radial from UMS.
The part itself is the Woodruff key, which keys the propeller drive to the crankshaft so torque is transmitted positively.
Why it is replaced: a sheared key is the usual result of a prop strike, and a worn key allows the drive to fret on the shaft. Supplied as a pack of 4, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Check the keyway in both the shaft and the drive for fretting before fitting a new key.
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