Replacement valve spring for the UMS 5-100cc, the five-cylinder 100cc four-stroke radial from UMS.
Function: it is the valve springs, which close the valve and hold it on its seat against the cam.
The reason this ends up on an order: springs lose tension with heat cycles, and a weak spring causes loss of compression and valve float at high rpm. Supplied as a pack of 10, which is how these small parts are normally bought — you rarely need just one.
Fitting note: Fit springs as a set — mixing free lengths across the engine is asking for uneven running.
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