This original O-ring seals the differential mechanism, preventing lubricant leakage and keeping contaminants out of the gear set. It sits in the groove between the differential housing and the mating cover or shaft. A degraded or hardened O-ring allows oil to weep past the seal, leading to low lubricant level and accelerated gear wear. This part is manufactured to the original cross-section and durometer, ensuring it compresses correctly into the groove without over-stressing the housing joint. On reinstallation, clean the seal groove thoroughly and verify the O-ring is seated evenly before tightening the housing bolts.