This is the piston for the UMS 5-55cc engine, the five-cylinder 55cc four-stroke radial from UMS.
The part itself is the piston, which transmits combustion pressure to the connecting rod and seals the combustion chamber against the cylinder wall.
Why it is replaced: pistons are replaced during a top-end overhaul, or after a lean running condition or detonation has scuffed the skirt. Supplied as a pack of 2, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Fit new rings with a new piston and check the bore for scoring before assembly.
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