This O-ring seals the gearbox housing at the shaft passage, preventing lubricant loss and keeping contaminants out of the drivetrain. It sits in the groove between the housing and the shaft, where it maintains a static seal under normal operating pressures. A degraded O-ring allows oil to weep from the gearbox, which can lead to low lubricant levels and accelerated gear wear. This component is manufactured to the original cross-section dimensions so it seats correctly in the groove without distortion. On reinstall, clean the groove surface and apply a light coat of gearbox oil to the ring before seating.