This gearbox O-ring seals the joint between the gearbox housing and its mating cover, preventing lubricant leakage and keeping contaminants out of the transmission. It sits in a machined groove on the housing face and is compressed when the cover is bolted into place. A degraded or hardened O-ring allows oil to weep past the joint, leading to low lubricant level and potential gear or bearing damage. The O-ring is manufactured to the original cross-section and durometer, which matches the groove geometry of the housing so that the seal compresses to the correct amount without being pinched or rolled. On reinstall, clean the groove thoroughly and apply a light coat of gearbox oil to the O-ring before assembly.