What does the pinion seal primarily seal in a differential?

• A. The gear teeth of the pinion
• B. The axle shaft where it connects to the wheel
• C. The pinion gear shaft where it enters the differential housing
• D. The differential cover

Answer: (C)

The pinion seal’s job is to keep lubrication inside the differential by sealing around the pinion gear shaft where it passes through the differential housing. As the pinion shaft and gear rotate, oil would leak outward along the shaft if there weren’t a tight seal at that junction. The pinion teeth themselves are inside the housing and are continuously lubricated by that oil, so they aren’t sealed by this seal. The axle shaft seal is a different seal that protects the shaft that drives the wheels, and the differential cover seal is used at the cover to keep oil from leaking at the cover edge. So, the primary function is to seal the joint where the pinion gear shaft enters the differential housing.