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An oil seal (shaft seal) is a ring that seals around a rotating shaft to keep lubricant in and contaminants out.

A flexible lip, held by a spring, rides against the shaft and seals the gap while the shaft turns, so oil cannot escape and dirt or water cannot enter.

Oil seals are fitted at the ends of shafts in the engine, gearbox, axles, and wheel hubs. The correct seal matches the shaft diameter and the housing bore.