This is the propeller drive for the UMS 7-77cc engine, the seven-cylinder 77cc four-stroke radial from UMS.
The part itself is the propeller drive, which transmits crankshaft torque to the propeller hub.
Why it is replaced: the drive is subject to the full propeller load and to any prop-strike shock, and a worn or cracked drive is a structural failure risk.
Fitting note: Inspect closely for cracks after any prop contact and check the key and keyway at the same time.
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