67 Mustang Posi Diff Swap Upgrade
67 mustang with an Explorer rear-end
If you haven’t already, I would highly suggest adding the 8.8 Explorer posi-trac swap on your short list. Check your local wrecker for any 1996 to 2001 Ford Explorers. Some of these came with tough posi-trac disc brake rear diff’s that just happen to be of the exact wheel base that works for a 67 68 mustang.
Very good bang for your buck.
Diff Code Chart
| 1996-2001 Ford Explorer door plate x-ref | |||
|---|---|---|---|
| Code | Description | Capacity | Ratio |
| 41 | Open 8.8” | 3200 | 3.27 |
| 42 | Open 8.8” | 3200 | 4.10 |
| 43 | Open 8.8” | 3200 | 3.08 |
| 44 | Open 8.8” | 3200 | 3.73 |
| 45 | Open 8.8” | 3200 | 3.55 |
| 46 | Open 8.8” | 3200 | 3.73 |
| D1 | L/S 8.8” | 3200 | 3.27 |
| D2 | L/S 8.8” | 3200 | 4.10 |
| D4 | L/S 8.8” | 3200 | 3.73 |
| D5 | L/S 8.8” | 3200 | 3.55 |
| L73 | L/S 8.8” | 3200 | 3.73 |
| L/S - Limited Slip Differential Open - Conventional Differential |
Find A Good Diff
After referring to this chart I was on the hunt for an L/S diff, thinking a D1 or D5 might be ideal. In all my travels, however I never saw a D1 or D5 axle in an Explorer. I did come accross a 96 Eddie Bauer Edition Explorer with fairly low mileage that had a D4 code on the door tag.
After confirming it hadn’t been swapped out with a single spin and that there were no metal chunks or visible damage I pulled it out of the Explorer and kept the brake hoses, lines and center hose along with the emergency brake cables attached.
Prep For Install
I cut the old mounts off the axle tubes and ground the areas clean, prepping them for new leaf spring perches to be welded on.
Picked up some new leaf spring perches (some people call them saddle mounts) from TFT OFFROAD (Car, Truck, Jeep, and Off-road Accessories). I got 2.5” wide ones for a 3” tube even though the 8.8 has 3.25” dia tubes, they fit fine. They were in stock and cheaper than buying online and paying for shipping, etc.
After removing the original rear differential I made sure to take the measurement from perch to perch from it. I took a sharpie and marked the clean tubes of the Explorer diff exactly where the perches should sit left to right to match the original center distance.
Test Fit
Using new U-bolts I test fitted the Explorer diff by just having it sit on the perches with a little tension on the U-bolts. I then lowered the car to achieve normal ride hieght. After this I used a floor jack to slightly turn the diff up in the front by 3 degrees to ensure it had the proper pinion angle.
(A driveshaft has limitations. Ideally the transmission output shaft and the pinion input shaft should be parallel, that means 3 degrees at each end. The angles are actually equal but opposite; 3 degrees downward at the transmission, 3 degrees upward at the pinion.)
Weld Perches
With a decent mig welder I tacked the perches in place then disassembled and pulled the diff back out. This allowed enough room to get a good quality weld from perch to tube and a coat of paint on the bare metal.
While I was at it I added a tab on each side so that I could bolt/secure the brake hoses.
Installation
Brake Line Adapter
Next I picked up a brake line adaptor to go from the 8.8 flex hose to the 67 stang hard line (7/16-24 inv. fem, to 3/8-24 inverted male). I went to NAPA BKP 6413307. Edelmann PN 258430 or Weatherhead 7826 should also work.
Drive Shaft
At the time of this swap/upgrade this 67 Mustang has a C4 automatic transmission. I kept the original driveshaft and used a 1310/1330 conversion u-joint to adapt it to the Explorer diff pinion flange.
It works, but there’s not really enough of a margin of safety on the pinion travel. I imagine if the car got airborn the pinion would ride the seal.
Another thing to be mindful of is the fact that the Explorer diff has an off centre carrier. It is well within the safety limits of allowable offset, but depending on the type of exhaust system installation it may be something extra to modify. In my case there was still enough clearance.
Fluid Change
After everything was ready I filled the diff with compatible LS gear oil. I went with 21300 Royal Purple 75W-90 Synthetic Gear Oil.
Results
It’s quite comforting to have updated tech like the rear disc brakes and the car is no longer a single tire roaster (like the 67 at the end of the movie InnerSpace). It hooks up great and has much more off-the-line power with the 3.73 gears. The C4 transmission likely has to be replaced with something that has over-drive. I still like to go on trips with the old girl.
Future Thoughts
Master Cylinder
A master cylinder meant for rear drum brakes will have a residual pressure valve to keep the mushroom seals sealed. Typically disc brake systems don’t have this residual pressure. The slight pressure may not affect anything or cause brake drag, but this should be looked into to confirm.
Driveshaft
To get the proper pinion/yoke travel there may be a few options.
- Shorten the existing driveshaft. (Looks like the original has a rubber core so probably should just be replaced)
- Get a new driveshaft made
- Find an 84 to 88 TBird driveshaft which is spec’d to be 49”
I will likely upgrade the transmission and decide then.
Emergency Brake Cables
Could use the parts from the Mustang with the original cables from the explorer using cable clamps (also called wire clips) or something. If so it would be 2 per splice. Again will wait and decide on the transmission upgrade and come up with something then.