Factory Five Roadster Build Part 6 - Rearend Install

If you ask your late-model Mustang friends, they'll tell you that 3.55s are the minimum gear to upgrade to (remember, these cars have Overdrive), and many go to 3.73s and even 4.10s. But these cars also have 3,500 pounds to push down the street and only 300 to maybe 400 lb-ft of torque with which to do it. When completed, our Factory Five Roadster will weigh around 2,200-2,500 pounds and will have over 500 lb-ft of torque. With these numbers in mind, DTS recommended a 3.27 ratio to match our power and our Tremec TKO-600 transmission gears. DTS carries several brands of gears, but Ford Racing Performance Parts 3.27s (PN FM M-4209-F327, $248) got the nod for this build.

The FRPP ring gear is placed on a set of press plates, and the Eaton differential is carefully seated with the press. Once seated, the new ring-gear bolts are installed and torqued to Ford's specs of 70-84 ft-lb.

The same press is used to push the new carrier bearings into place with the aid of a special-sized spacer to press on the inner race of the bearing only. This prevents warping the cage or damaging the bearing rollers.

The same steps are performed on the pinion gear and its new bearing.

If you've ever watched someone work on a Ford differential, you've probably seen an item called a crush sleeve being installed. A crush sleeve is installed on the rear of the pinion gear to set pinion depth and helps set up pinion bearing preload when the pinion nut is torqued to the proper specs. The problem is, on high-powered cars with locking or high-bias differentials, they place more load on the pinion crush sleeve than normal and will actually compress it further, changing pinion depth. A solid crush sleeve (PN IK 4105, $16.32) prevents this distortion, and the pinion depth and preload is set by shims.

After installing the pinion gear and setting pinion depth, the differential and its mounted ring gear and bearings are placed into the housing with a preliminary shim pack. Gear-marking compound is used on the ring-gear teeth, and the ring gear is inspected for the proper pattern and backlash via a dial indicator.

If the ring-gear backlash is out of spec, the carrier shims are adjusted from side to side, and the pattern can be further adjusted by changing the pinion depth.

Once the crew at DTS gets the backlash and pinion depth finalized, the carrier bearing caps are torqued to their factory specs of 76-89 ft-lb. DTS can optionally install carrier bearing studs and billet steel caps for even more strength.

Here, DTS uses Strange Engineering 31-spline S/S series alloy axles. New axle bearings are pressed on before the axles are installed. The 9-inch bearing ends utilized on our housing require 9-inch-style bearings, of course. The 9-inch-style bearing is superior to the stock 8.8-inch-style bearing for two reasons: One, the bearing is simply larger and can take more load, and two, the bearing has a true inner race whereas the 8.8-inch axle bearing uses the axle shaft itself as an inner race. This means if the bearing goes bad on a stock 8.8, it usually takes the axle with it. Ouch.