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

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2003-2004 Mustang Ford 31 Spline 8.8" Differential

This Ford Differential was stock in 2003-2004 Cobra Mustangs, but will fit any Mustang with a 31 Spline 8.8" rear end.

This 31 Spline 8.8-inch differential is the stock replacement if the differential were to fail in your 2003-2004 Mustang Cobra. However, if you have any Mustang with a 31 spline 8.8-inch rear end, you can upgrade to this differential, which features traction lock and carbon clutches. This installation also covers a complete rear end upgrade, including axles, bearings and seals and wheel studs.

This Install Also Applies To:

• Mustang Rear Axle Shaft Yukon 31 Spline 5-Lug Without ABS Ring 1999-2004
• Mustang Ford Racing Ring And Pinion Install Kit Improved Deluxe 8.8" 1986-2013
• Mustang 8.8" Rear Axle Bearing & Seal Kit 1986-2004
• Mustang Wheel Studs Rear 3" 1979-2009

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:

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• 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.

2. Jack up the car, supporting the frame with two jack stands and letting the rear end hang.

3. Remove the Mustang wheels.

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.

5. Remove the clip holding the parking brake cable to the caliper with needle nose pliers or a screw driver.

6. Remove the Mustang brake caliper with a 12mm socket.

7. Support (hang) the caliper so the Mustang brake hose doesn’t get damaged.

8. Remove the caliper bracket with a 14mm socket.

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.

10. Remove the brace with a 13mm socket.

11. Remove the ABS sensor bolt with an E8 Torx.

12. Remove the dust shield with an 8mm socket.

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.

14. Pull the driveshaft out of the Mustang transmission and it set aside.

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.

16. Keep track of all of your bolts as you will have quite a few.

17. Remove the bottom bolts from the Mustang differential cover and loosen the uppers with a 15mm socket.

18. Use a pry bar to slowly drain the fluid. Then fully remove the cover.

19. Spin the differential until you can easily access the bolt holding the pin in the differential.

20. Use an 8mm wrench to remove the bolt, as there will not be room for a socket.

21. Spin the differential so the pin will slide out.

22. Spin the differential again so you will be able to access the Mustang rear axle retaining clips.

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.

24. Use a magnet to remove the rear axle retaining clips. You may need to spin them.

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:

26. Flat side:

27. Carefully remove the Mustang rear axles. Support them so they do not drag through the axle tube.

28. Push the ABS sensor out of the bracket if you have not already.

29. Remove the bracket with a 14mm socket and wrench.

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.

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.

32. Now you can use a seal puller to remove the Mustang differential pinion seal. Be sure to not damage the washer behind it.

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.

34. Remove the ring gear from the differential with a 19mm socket.

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.

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.

37. Old bearing:

38. Remove the old pinion races with a hammer and punch. Install the new races with a race driver.

39. Slide the Mustang pinion crush sleeve on the end of the pinion gear:

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.

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.

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.

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.

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.

45. Use a large socket to install the new Mustang axle bearings and seals.

46. Now you can move to the exterior of the rearend and install the new Cobra brackets.

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.

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.

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.

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!