Ford Mustang rear pinion flange leak due to groove, requiring flange and pinion seal replacement on Ford Mustang

Some Ford Mustang owners report experiencing rear pinion flange leaks, often caused by damage to the sealing surface of the pinion flange, such as nicks or grooves. When this occurs, it may be necessary to replace both the pinion flange and the pinion seal, or to use a repair sleeve to fix the sealing surface. While replacing the pinion flange seal is generally a straightforward task that can be completed in about an hour, it requires the right tools and involves removing the drive shaft to access the seal. Additionally, some owners find that a bent pinion flange can lead to excessive runout, necessitating the installation of a new pinion. Symptoms of a leaking pinion seal include gear oil seepage around the pinion flange area, which can eventually run down to the differential cover, making gear oil visible on the body above the pinion flange. Prompt attention to these issues is essential to prevent further leaks and potential damage.