This O-ring seals the interface between the cutterbar housing and the disc-drive assembly on a disc mower. It prevents hydraulic oil or gear-case lubricant from leaking past the rotating shaft or stationary joint. A degraded or pinched O-ring allows fluid loss, which leads to bearing starvation and eventual drive-component seizure. The cross-section and inner diameter match the original seal groove so that the ring compresses evenly when the mating flange is torqued to specification.