Mustang Ford 31 Spline 8.8" Differential (2003-2004) Installation Instructions

Tools Required:
  • 3/8-inch & 1/2-inch Ratchet
  • 13/16-inch Socket
  • 1-1/16-inch Socket
  • 8mm Socket
  • 12mm Socket
  • 13mm Socket
  • 14mm Socket
  • Numerous Extensions
  • 8mm, 14mm Wrenches
  • 1/2-inch Impact Gun
  • E8 External Torx Bit
  • Pliers
  • Jack & Jack Stands
Install Difficulty:
  • Difficult
  • Difficult Project
1. To start installing your new Mustang Ford 2R3Z-4026-AA 31 Spline 8.8-inch Differential, loosen the wheels with your 13/16-inch socket.
Mustang Ford 31 Spline 8.8
2. Jack up the car, supporting the frame with two jack stands and letting the rear end hang.
Mustang Ford 31 Spline 8.8
3. Remove the Mustang wheels.
Mustang Ford 31 Spline 8.8
4. Remove the emergency brake cable end using vice grip pliers. Make sure not to damage the Mustang rear parking brake cable bracket. Be sure the parking brake is off.
Mustang Ford 31 Spline 8.8
5. Remove the clip holding the parking brake cable to the caliper with needle nose pliers or a screw driver.
Mustang Ford 31 Spline 8.8
6. Remove the Mustang brake caliper with a 12mm socket.
Mustang Ford 31 Spline 8.8
7. Support (hang) the caliper so the Mustang brake hose doesn’t get damaged.
Mustang Ford 31 Spline 8.8
8. Remove the caliper bracket with a 14mm socket.
Mustang Ford 31 Spline 8.8
9. Remove the Mustang brake rotors. The Cobra brake upgrade kit includes new rotors so you can use a metal hammer to remove the old ones. If you’re reusing your rotors, use a rubber mallet. Spray some lubricant around the wheel studs and hub center to help break the rotor loose.
Mustang Ford 31 Spline 8.8
10. Remove the brace with a 13mm socket.
Mustang Ford 31 Spline 8.8
11. Remove the ABS sensor bolt with an E8 Torx.
Mustang Ford 31 Spline 8.8
12. Remove the dust shield with an 8mm socket.
Mustang Ford 31 Spline 8.8
13. Put the car in neutral so the Mustang driveshaft can spin easily if you're using an impact gun. Remove the driveshaft bolts at the rear end with a 12mm 12 point.
Mustang Ford 31 Spline 8.8
14. Pull the driveshaft out of the Mustang transmission and it set aside.
Mustang Ford 31 Spline 8.8
15. Loosen the Mustang pinion nut with the 1-1/16-inch socket. Be sure to catch the pinion flange, as it may fall off once the nut is removed.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
16. Keep track of all of your bolts as you will have quite a few.
Mustang Ford 31 Spline 8.8
17. Remove the bottom bolts from the Mustang differential cover and loosen the uppers with a 15mm socket.
Mustang Ford 31 Spline 8.8
18. Use a pry bar to slowly drain the fluid. Then fully remove the cover.
Mustang Ford 31 Spline 8.8
19. Spin the differential until you can easily access the bolt holding the pin in the differential.
Mustang Ford 31 Spline 8.8
20. Use an 8mm wrench to remove the bolt, as there will not be room for a socket.
Mustang Ford 31 Spline 8.8
21. Spin the differential so the pin will slide out.
Mustang Ford 31 Spline 8.8
22. Spin the differential again so you will be able to access the Mustang rear axle retaining clips.
Mustang Ford 31 Spline 8.8
23. Push the axles towards the center of the rear end to expose the c-clips inside the differential. This will also push the Mustang ABS sensors half way out. Then they can be easily removed.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
24. Use a magnet to remove the rear axle retaining clips. You may need to spin them.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
25. One side of the rear axle retaining clip is perfectly flat and the other is a little rounded. The flat part faces the outside of the car and the rounded side faces the center of the differential. Rounded side:
Mustang Ford 31 Spline 8.8
26. Flat side:
Mustang Ford 31 Spline 8.8
27. Carefully remove the Mustang rear axles. Support them so they do not drag through the axle tube.
Mustang Ford 31 Spline 8.8
28. Push the ABS sensor out of the bracket if you have not already.
Mustang Ford 31 Spline 8.8
29. Remove the bracket with a 14mm socket and wrench.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
30. Remove the differential caps with a 19mm socket. Be careful. Once all four bolts are removed, the differential can fall out. Also, be sure to keep all of the Mustang differential bearing shims in the same place they were. You must pull these out with the differential.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
31. Hammer out the pinion gear. Be sure not to mess up the threads. You can put a ball peen hammer on the end of the pinion gear and hit the back of that hammer with another hammer. Use something to catch the pinion as it can fall out of the differential housing, or have someone hold onto it.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
32. Now you can use a seal puller to remove the Mustang differential pinion seal. Be sure to not damage the washer behind it.
Mustang Ford 31 Spline 8.8
33. It's clear that my differential was bad because the differential bearings popped off by hand (They should be pressed on/off). Both the bearings and differential have pitting and grooves, which caused a loud whine coming from the rear end.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
34. Remove the ring gear from the differential with a 19mm socket.
Mustang Ford 31 Spline 8.8
35. Remove the pinion seal, thin washer, bearings, and races from the differential housing. Be sure you don’t lose the washer from behind the pinion seal. If you don’t reinstall it, you’ll have a leak.
Mustang Ford 31 Spline 8.8
36. Remove the bearing from pinion gear and install the new Mustang differential pinion bearing. A hydraulic press will be needed. There is a shim under the bearing on the pinion gear so be sure not to lose it.
Mustang Ford 31 Spline 8.8
37. Old bearing:
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
38. Remove the old pinion races with a hammer and punch. Install the new races with a race driver.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
39. Slide the Mustang pinion crush sleeve on the end of the pinion gear:
Mustang Ford 31 Spline 8.8
40. Push the pinion gear into the rear end. Slide the front pinion bearing on, followed by the washer, then the Mustang differential pinion seal. Lightly tap on the seal until it is fully seated.
Mustang Ford 31 Spline 8.8
41. Install the pinion flange and pinion nut. The nut must be torqued on. You will need an inch/pound torque wrench with a dial to check the force required to turn the pinion. As you tighten the pinion nut, you crush the crush sleeve. The more it becomes crushed, the harder it will be to spin the pinion flange by hand. Without the correct pre-load, the bearings can be damaged.
Mustang Ford 31 Spline 8.8
42. Press the bearings on either side of the differential with a press and install the ring gear. Once assembled, coat the ring gear with the supplied paint to check your pinion to ring gear clearance. You want to see the mark right in the middle of the gear.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
43. Install your differential with the shims that were previously used for a starting point. In my case, the original shims worked just fine. If needed, use the supplied shims to get your differential in the right position. Apply more paint and spin the differential again. When checking clearance, simply snug up the caps. There is no need to torque them to spec yet. There is a special tool that makes installing the shims easy.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
44. Once the Mustang differential is fully installed, you can replace the old axle bearings and seals. Use a seal puller if possible. I used a hammer and chisel and hit outward on the lip. I then used a three finger puller for the bearing.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
45. Use a large socket to install the new Mustang axle bearings and seals.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
46. Now you can move to the exterior of the rearend and install the new Cobra brackets.
Mustang Ford 31 Spline 8.8
47. Your new Mustang Yukon rear axles will need wheel studs and ABS rings. I had the ABS rings pushed off my old axles and onto my new axles. Be sure to measure the space between the front of the hub on the axle and the ABS ring. The new Moroso 3” extended wheel studs were pressed onto my new axles.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
48. Install the axles. Push them in as far as they will go so you can install the retaining clips. Once the retaining clips are installed, pull the axles out and make sure they seat.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
49. Once the axles are in place, you can install the differential cover. I used the supplied Mustang differential cover gasket, but RTV can be used as well. Just make sure you have an extremely clean surface to apply the RTV. I installed the two tags as well.
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
Mustang Ford 31 Spline 8.8
50. Install the rest of the brake parts the opposite way you removed them. The rear end should be filled with a bottle of friction modifier and differential fluid through the fill hole until fluid starts to come out. At this point, you're done installing your new Mustang Ford 31 Spline 8.8-inch Differential!