Jim Smart
June 23, 2014

Few things are more annoying than seal leaks of any kind. They stain the driveway and lose all-important lubrication at the same time. Even more frustrating is replacing a seal in a differential that continues to leak. How do you replace a seal and stop the leak once and for all. And—what else do you need to know about rear axle seal replacement while you're in there? When you replace a pinion seal, you also need to think about axle shaft bearings and seals because if the pinion seal is leaking chances are good all seals probably require replacement. Our subject vehicle is a '04 Mach 1 with the 8.8-inch integral carrier rear axle, which is common to '85-'14 Mustangs and many models of Explorer, Ranger, F-series, and more. We're also seeing more restomod builds with the corporate Ford 8.8-inch housing under them as well, which makes this repair procedure quite important to Mustang owners. The Mach has a leaky pinion seal we need to replace. However, it has a lot of miles on the odometer, which means there are other items that are likely worn out as well.

Our belief about rear axles is this: They mandate regular lube changes just like the engine and the rest of your driveline. Rear axle lube should be changed every 30,000 miles along with a transmission service. Of course those of you with 8- and 9-inch removable carrier differentials have the challenge of how to drain the lube. Your only option is to siphon the dirty lube out and replenish with fresh lube via the filler port. Those of you with integral carrier axles get to remove the axle housing gear cover.

Successful seal replacement boils down to sweating all the details including things we don't ordinarily notice like seal contact surfaces, seal lip condition (on a new seal), seal garter spring status, and seal perimeter condition. Scoring on seal lip contact surfaces will cause a leak. Loss of the garter spring will cause a leak. Failure to lube the seal lip and contact surfaces will cause seal damage during installation and possible leakage. Seal replacement cannot be performed with reckless abandon and hurry. You must be methodical in every aspect of your work regimen. When you are replacing a pinion seal, everything must go back together exactly as it came apart, meaning flange yoke nut torque and crush sleeve thickness. If you don't, you can wind up with unnecessary noise and most likely excessive ring and pinion gear wear that will ultimately lead to failure.

A Word About Pinion Preload

The factory used crush sleeves on pinion gears to achieve an acceptable preload during axle assembly at Ford's Sterling Height, Michigan axle plant. Performance rebuilders generally use a solid sleeve with shims to get the desired pinion preload. When you replace a pinion seal, you want the same preload, which calls for the nut to pinion alignment marks shown earlier. We've also seen installers count the number of turns to remove the pinion nut and then reinstall the nut the same number of turns, lining up the marks on the last turn. If your 8.8 has already been rebuilt, chances are good it has the solid sleeve with shim stock. In any case, go with nut to pinion alignment marks to get pinion preload where it was prior to disassembly. Manually turn the companion flange. If it is too tight, back off and try again.

01. Tell tale signs of pinion seal leakage are seen here. Very slight dampness is no cause for alarm; leakage like this is reason for seal replacement. Note the large vibration dampener on this 8.8-inch rear axle, which was incorporated on some of these rear ends to reduce driveline harmonics.

02. Pinion seal replacement begins with driveshaft removal using a 12mm 12-point socket. Do you see the yellow reference marks on the yoke and driveshaft? Use those as alignment marks for reinstallation. If there are no marks, mark both the yoke and shaft with a grease pencil, touch up paint, or a scribe.

03. Mark the companion flange locknut and pinion as a reference. Remove the pinion companion flange retaining nut using a 11⁄16-inch socket.

04. The pinion yoke is removed using a wheel puller as shown. Protect the end of the pinion gear against damage.

05. The pinion seal is removed using a seal remover. Alternatively you can carefully pry the seal out of the housing using a common screwdriver.

06. This is a typical pinion seal with a garter spring around the inside lip designed to maintain a positive seal at the yoke sleeve. Seals fail when this spring pops out. Prior to installation, pack this seal lip with wheel bearing grease, which will ensure garter spring retention. Apply a very thin film of Permatex’s The Right Stuff or equivalent around the outside perimeter, which provides a leak barrier at the casting.

07. Before you seat the seal, clean the seal cavity thoroughly. Examine contact surfaces for any scoring or debris before seating this seal. Give it a gentle love tap to get the seal started, and then use a seal driver to uniformly seat the seal flush. Make sure the garter spring remains in place.

08. Next is the seal protector or debris shield, which keeps dust and dirt away from the seal lip.

09. The companion flange is cleaned and inspected for scoring. Any scoring here is unacceptable because it will damage the seal and start the leak process all over again. Dress the surface with emery paper or replace the yoke as needed. Lubricate the flange with assembly grease.

10. Gently seat the companion flange on the pinion using your reference marks.

11. The flange retaining locknut gets RTV sealer, which keeps gear lube from weeping along the threads and creating a leak.

12. The flange is tightened with an air impact wrench and then torqued until the reference marks line up. This ensures proper pinion sleeve crush and preload.

13. Reinstall the driveshaft using the alignment marks made earlier. The driveshaft yoke bolts receive fresh thread locking compound and are torqued to 70-95 ft-lb in a crisscross fashion. Top off the axle with 75/140 gear lube and a friction modifier for the Traction-Lok unit.

14. While you’re replacing the pinion seal, seriously consider replacing axle bearings and seals to complete this service visit.