2002 Ford Mustang GT 8.8 Rearend Rebuild - Rearend Revival

To confirm backlash is good, Chris coats the ring gear with gear-marking paint in two separate areas, and then rotates the ring past the pinion a few times to establish a wear pattern. Backlash is the amount of clearance between the pinion and ring gear, and the source of the roaring type of sound that is sometimes heard coming from the rearend during deceleration (too much backlash).

Note the position of the pattern between the face and flank of the teeth. Since the pattern is centered from face (the top land of the tooth) to flank (the base of the tooth) on the Drive side, the pinion depth for our rear is good to go. If the contact pattern had been more toward the face of the teeth on the ring gear, the pinion would have to be moved closer toward the ring-gear's centerline by adding pinion shims. Conversely, if we had seen a pattern that was closer to the flanks of the marked teeth, shims would have to be adjusted so the pinion is moved away from the ring gear's centerline.

A great deal of trial-and-error goes into building and dialing-in a fresh 8.8. Our Detroit Locker requires side shims before it can be final-installed. Determining correct shim sizes (thickness) takes a while because the locker must be installed and removed several times. However, nailing the perfect shim combination allows Chris to determine the amount of backlash and preload is necessary to absorb the carrier's side-to-side movement on the ring gear. A good, tight fit in the housing is important for any differential.

Setting pinion-gear depth is another test-fit (using a test bearing), adjust, and refit exercise that culminates with a new rear pinion bearing being pressed onto the pinion shaft. Chris initially tries shims of various diameters, before settling on 0.030 inch as the correct size for proper pinion depth.

This is the front pinion bearing and seal package. A shim is placed between these pieces, and serves as a slinger, of sorts, that meters the amount of gear oil that actually reaches the seal. Chris recommends using gear oil on all of the bearings and grease for the seals in a freshly built rearend.

Each bearing cap is marked with a set of arrows that must point toward the outside of the rearend housing...

...Once caps are installed, Chris tightens each fastener with 60 lb-ft of torque.

Pinion-bearing preload is a measure of the pinion bearings' rolling resistance, once the pinion nut is fully torqued. Chris sets preload carefully at 15 in-lb.

Having the correct amount of preload on the carrier bearings prevents the locker from moving-and thus increasing backlash on the gears-when our 'Stang is flogged. Before calling this project done, Chris checks backlash by mounting a dial indicator on the rear and setting the plunger perpendicular to the teeth on the ring gear, then rotating the ring gear back and forth for measurements. As a rule of thumb, backlash changes roughly 0.007 inch for each 0.010 inch that a differential is moved, either closer to (decreases) or farther away from (increases) the pinion gear's centerline. Chris says from 0.008 to 0.012 inch is OK, but new gears like our 4.30s should be set up more on the tight side, so the backlash will still be within that range after a 100-mile break-in period.

Stock axles just don't cut it when you start making real steam with your Mustang. Our Pony wears Strange Engineering's SS Series C-Clip (31-spline) street axles, which, despite actually having C-clips, are the same induction-hardened, forged axles as other axles in Strange's S-series. We selected these axles to maintain our 'Stang's OEM-style rear braking hardware.

Chris pours Royal Purple 75W-140 gear oil into a gallon-size feeder container, from which the fluid is pumped into the differential. Royal Purple is plenty slick, helps keep things quiet in the housing, and its Synslide additive protects the internal pieces of clutch- and cone-style differentials, if you're using one of those instead of a locker.

It don't mean a thing if it ain't got that bling, right? After installing a new gasket (which is supplied), Chris put UPR's beautiful, new billet differential cover over our Pony's updated gearset and locker. The stout, polished lid features bearing supports that keep caps in place, and help maintain pinion depth and backlash.