This O-ring seals the connection between the PTO shaft and the housing, preventing hydraulic oil or gearbox lubricant from leaking at the coupling point. It sits in the groove on the output side of the PTO assembly, where it compresses against the mating flange to maintain a closed circuit under pressure. A degraded or flattened O-ring allows fluid to weep past the seal, leading to a drop in system pressure and eventual clutch-slip or engagement delay. As an original AGCO component, the cross-section and durometer are matched to the groove geometry so the seal compresses to the correct squeeze without extruding. On reinstall, ensure the groove is clean and free of old sealant, and lubricate the O-ring lightly before assembly to prevent rolling.