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Gear installation

3,803
3,745
So a buddy has a 7.5 diff that will be getting a 3.73 ring and pinion. The 7.5 is a close relative to the 8.8, so the procedure will be essentially the same. The little 7.5 was around until 1992 ish in the Gt and works well with engines up to about 350HP.
Since Ford Performance no longer makes gears for this diff, I was forced to go aftermarket with a Yukon gear ring and pinion, plus an install kit. I always prefer the FP stuff for reasons that will become apparent shortly.
I made a tool for this project, which can be found here: https://trackmustangsonline.com/threads/pinion-depth-tool.18044/#post-269780 and will be using ti to install the new parts.
First off, the diff was removed from the car so I could work on it easier, this is not the only way to do this, but with my back issues, it was the simplest way to negotiate the gear change.
A cursory check of the pinion depth was done, it was 2.311.
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At this point I read, ( or rather tried to read) the pinion depth on the new gear..
it looks like 1.709.. or something, which was seriously.. very very weird.
1FR88Dil.jpg

So, with that, I called Yukon gear to see if they could shed any light on the problem, what I got was, "use a .025 shim and it should be close".. not really enamored with that answer, since this means I'll be chasing the gear pattern, which means I'll be taking this apart a couple of thousand times to change the pinion shims.
Which leads to outlaw mod #1
Since installing the inner pinion bearing several times over means there is a very real possibility of damaging the bearing due to it's pressed fit, and.. I don't live in an area where parts are easily obtained if this should happen, so.. I talked to a dragster friend of mine who suggested that I sand down the pinion in order to make the bearing a slide on, rather than a press fit. Although, I've heard of this, I've never actually done it, and have actually recommended AGAINST it, but he was adamant that it would work without issue as long as I didn't get too aggressive in removing the material. So, a few laps in the lathe with 80 grit sandpaper, and we now have a slip fit where the press fit used to be. It is still difficult to get on, it really has to be dead straight, but it will slide on. So off we go, hoping this will turn out well.
Sometimes you have to take the bull by the tail, and face the situation..
Tkusokwl.jpg

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So, the diff was cleaned up and prepped for the next installment, I also noticed one of the bushings trying to escape, that I will have to fix.
Qf4pl1dl.jpg
 
3,803
3,745
What a PITA, If finally chased the pinion depth down to where I could live with it, after using the calipers on the old pinion, and sort of coming up with a shim thickness. This of course means assembling and disassembling the diff at least 3 times, while searching for a pinion depth on the ring gear. I did it all with the old crush sleeve, no seal and the old nut, until I got serious with it.

OPzIVHal.jpg

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Once that was done, I installed the limited slip unit and ring gear until I got the .011 back lash and a decent gear pattern, sounds easy, took all day.

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looks pretty decent, hopefully there will be no issues, goes back in tomorrow.
 

xr7

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Exp. Type
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Exp. Level
5-10 Years
Minnesota
I feel you pain. Absolutely hate integral carrier diffs. Would rather do a clutch job on a stinking garbage truck.
 
I just finished doing this exact job on my S197. Installed a Torsen T2R and 3.73 ratio change. Did the same approach, made a set up bearing (honing out the ID on a new bearing) and honed in on the pinion depth via pattern. 11 thou backlash gave a perfect pattern and was within spec.

Fast forward to yesterday and I ordered the Ford Rotunda dealer pieces on eBay required per the Ford instructions and I am going to redo the job. I don’t know how, but there is too much backlash on power on/off modulation and coasting in neutral on the highway. While doing the install I had a configuration that landed at 9 thou with the same pinion shim but the pattern told me the backlash was too tight even though the backlash is within Ford spec of 8-12 thou.

When I redo this I’m hoping to get the exact pinion shim right and aim for 8 thou backlash to keep it tight. I’m also getting a carrier shim set because setting backlash was a humongous pain with the provided shims in the kit. Ended up lapping shims to get the right thickness while maintaining carrier preload.

Im not looking forward to redoing this job…
 
I just finished doing this exact job on my S197. Installed a Torsen T2R and 3.73 ratio change. Did the same approach, made a set up bearing (honing out the ID on a new bearing) and honed in on the pinion depth via pattern. 11 thou backlash gave a perfect pattern and was within spec.

Fast forward to yesterday and I ordered the Ford Rotunda dealer pieces on eBay required per the Ford instructions and I am going to redo the job. I don’t know how, but there is too much backlash on power on/off modulation and coasting in neutral on the highway. While doing the install I had a configuration that landed at 9 thou with the same pinion shim but the pattern told me the backlash was too tight even though the backlash is within Ford spec of 8-12 thou.

When I redo this I’m hoping to get the exact pinion shim right and aim for 8 thou backlash to keep it tight. I’m also getting a carrier shim set because setting backlash was a humongous pain with the provided shims in the kit. Ended up lapping shims to get the right thickness while maintaining carrier preload.

Im not looking forward to redoing this job…
My patterns, for your reference. The gears are nice a quiet. They just slap on throttle modulation which sucks. If they didn’t, I would leave it. No gear whine.

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3,803
3,745
before you tear that back apart, make sure you have no driveshaft slop, especially if you still have the center bearing style driveshaft. I'm not sure that clunk is from the ring and pinion.
 
I have the aluminum FRPP shaft, so it’s not that. Thanks for the suggestion though.

I changed diff fluid and it made a difference in the tone of the sound so I know it’s either the backlash in the gears, or perhaps excessive play between axle splines and the T2R. But I have never heard of the diff/axle spline interface being an issue.

This isn’t a fun problem to have after spending so much time and taking so much care to make sure everything was set up properly and within spec. This was my first go at this. Maybe experience would have told me to aim for the low end of backlash to avoid this sound. Would have caused me to pull the pinion bearing and find a new shim that gets me a good pattern at the tight end of the backlash spec. That’s what I will be shooting for by redoing the install with the Ford tool set
 
3,803
3,745
I have the aluminum FRPP shaft, so it’s not that. Thanks for the suggestion though.

I changed diff fluid and it made a difference in the tone of the sound so I know it’s either the backlash in the gears, or perhaps excessive play between axle splines and the T2R. But I have never heard of the diff/axle spline interface being an issue.

This isn’t a fun problem to have after spending so much time and taking so much care to make sure everything was set up properly and within spec. This was my first go at this. Maybe experience would have told me to aim for the low end of backlash to avoid this sound. Would have caused me to pull the pinion bearing and find a new shim that gets me a good pattern at the tight end of the backlash spec. That’s what I will be shooting for by redoing the install with the Ford tool set
Just for fun, recheck the u joint bolts, this car I'm working on, when I went to remove the diff, the bolts were hand tight, same with all the suspension stuff, although the fluid change sounds pretty condemning though.
 
Ford Rotunda set turned up…

Have a new crush sleeve, pinion nut and pinion seal on the way as well. Will probably tear it all apart in a couple of weeks. I’ll report back with final result so I don’t clutter your thread up.

7E6F8A65-4E2B-4DFF-A0D1-3871ADDCB324.jpeg
 

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