Fox Body Rear End Swap Problems
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Old August 3rd, 2013, 02:19 PM   #1
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Fox Body Rear End Swap Problems


Hey guys, I have a 1991 Mustang GT that I recently upgraded the rear end in. The rear end that was in it was the stock 8.8 rear end that had 2:73 gears (hence the upgrade). It had drum brakes as well.

This is where things get a little complicated. I had bought a parts car awhile back, a 1988 Mustang GT. The reason for buying it was because it had a 8.8 rear end in it with 3:73 gears and the price was reasonable to buy the rest of the car as well. So I bought the car and brought it home and started stripping parts off of it.

Just recently I finally got around to installing the rear end that was in that parts car into my actual driver (the 91 GT). The rear end that came out of the parts car had disc brakes instead of drum brakes, but still had 4-lug axles.

The rear end is currently installed in my car and it functions, but there are a few problems. First of all, the quad shocks that were the stock ones that go along with the stock rear end are no longer usable. We drove it down off the ramps and noticed the back end sat up real high and was as stiff as a board, so we got it back up in the air and found that the quad shocks were maxed out, so we took them out so that the rear suspension could actually function.

However, now that the quad shocks are out, the tires ( 17" rims, 255/45's) now rub going down the road quite a bit. We've been looking at it and it seems as though the rear end we put in is a little bit wider than the stock one that was in it. We've been thinking that the rear end with the 3:73's was out of a SN95 Mustang, which is why it had disc brakes, but then why would it still have 4 lug axles?

Another issue we have is that the e-brake cables don't reach quite far enough in this new rear end, they seem to be an inch or so too short. So do you guys have any suggestions on what e-brake cables would work?

So if any of you guys can shed some light on my problems that would be great. I basically just need to know what to do about the rubbing issue due to the wider rear end. I think it could be solved with new quad shocks, but I don't know which ones to buy due to not knowing what the rear end I have is originally out of, or if they even make quad shocks that would work. The issue seems to be where the quad shock bolts up on the rear end itself. On the stock fox body rear end the bracket was straight up and down, on this new one the bracket is at a bit of an angle, which is why the quad shocks don't work anymore.

Thanks in advance for taking your time to read this and help me out, I appreciate it greatly.

(ps. do any of you know right off hand which speedometer gear I need for the 3:73 gears so that my speedo is accurate?)
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Old August 3rd, 2013, 03:00 PM   #2
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Sounds like a Thunderbird Turbo Coupe Differential, so it would be about .75in wider on each side. Also the mounts need to be fitted for the quad shocks on the Mustang.
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Old August 3rd, 2013, 04:09 PM   #3
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New quad shock are not going to solve your rubbing. If your tires were not rubbing with them on it is only because they were maxed out binding your suspension and holding the back of your car up in the air? Ride height and fender clearance has nothing to do with the quads. They are Ford's band-aid to limit wheel hop. If you are worried about wheel hop and do not want the quads, get just about any aftermarket rear control arm, the wheel hop will be reduced by at least the same amount that the quad shocks were capable of and your quad shocks can stay in the trash. That still leaves you with rubbing issue, get rims with more backspace, narrower tires, or roll your fenders?

Pulling an axle and measuring it would give you a better idea of what is in there, it is probably a TC or could be a mixture of things like a 95+ Mustang with 4 lug axles.
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Old August 3rd, 2013, 05:38 PM   #4
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Sounds like you definitely have an '87/'88 Thunderbird Turbo Coupe rear axle. They came factory with 3.55 gears (manual trans) or 3.73 gears (automatic trans) and 4-lug disc brakes. The identical brake set-up was also used on the 1993 Cobra. The axle shafts in these rear ends are 3/4" longer for a 1 1/2" overall track width increase when installed in a '79-'93 Mustang.

Ford Motorsport / Ford Racing offered these same brakes in kit form (M-2300-C) for use on Mustang's but featured special caliper mounting brackets with an offset in them so that the Mustang's stock axles could be re-used and the stock track width retained.

The Ford kit is no longer available but North Race Cars currently produces similar brackets and they are of good quality. Removing the existing caliper brackets and installing the North Race Cars brackets would allow you to keep the disc brakes and restore the track width to stock and therefor resolve your tire rubbing issue.

The brackets are available directly from North Race Cars or Late Model Restoration sells them. Here is a link to LMRS:

1979-93 Mustang Rear Disk Brake Caliper Adapter Brackets for Use with 1987-88 Thunderbird Turbo Coupe Calipers with Hardware - LatemodelRestoration.com

The axle damper brackets on the Thunderbird rear axle are different so they will either need to be modified or you will have to go without the axle dampers.

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Old August 5th, 2013, 05:13 PM   #5
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definatly turbo coupe rear end. JUST did this and had exact same problems. take the quad shocks off and drill the hole's for them on the dif. 2inch's lower. and make sure you didn't tighten your control arms with the rear end hanging. tighten them as close to ride height as possible. this will fix all your problems, I litterilly just did this last weekend. You don't need any special brake brackets or anything but brake lines, the cables part#LRS-2635D form late model resto. and a 94-95gt master cylinder and proportioning valve. as long as you don't mind the width you'll be done.
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