GM LSD Underway

[MerkXRTurbo]

Hey all, sorry I've been away for a while. Been a bit busy with non car related stuff. Anyway, this weekend I've had a chance to do some work on my GM LSD, and have some pics to go along with it. It's not complete yet, as I will have to have some spacers machined in order to get it to fit right in the XR's differential housing, and I also have to have the diameter of the stub axles turned down in order to fit (not the splined portion, obviously). I'll explain both of these in better detail at a later date. BTW, in case anyone was wondering, I got the GM carrier from a friend, and he remembers getting it from a 1986 Trans Am, but there is always a posibility that his recollection is incorrect.

Comparison of the carriers (XR open differential left, GM LSD right). You might notice that the GM axle hole is ever so slightly smaller than the XR's, this the reason that I will have to have the stub axles turned down:





Here are pics of all the markings found on the GM carrier... not sure if any of these are significant to anybody:









Comparison of the ring gears (XR left, GM right). As you can see, both are 7.5" (I have heard rumors of some GM rings being 7.65"):







The only markings located on the GM ring gear... not sure if these are significant to anybody either.....



Look, no modification is required to bolt the XR ring gear to this GM carrier!!! The holes line up perfectly!!!





Shot of the carrier with the passenger side bearing race removed, which is required (in addition to grinding) to get the carrier into the XR's pumpkin. Due to the extra size of the LSD carrier it's impossible to turn it at the correct angle inside of the case in order to get it installed, so clearance must be made. I never understood the need for grinding until I attempted to install it 1st hand.



I don't have all the answers yet, as I am still gathering information and figuring out what I need to do to complete the project. It all lies within the spacers (might need one under the bearing race and under the ring gear also, but have not yet determined) and the stub axle machine work.

[Tumbler]

Merk You rule keep us updated I for one can't wait to do this swap

[anglin]

I don't have all the answers yet

You have definitely advanced our knowledge with these pictures. I can't wait to see the following pictures and read your write up on this. It should be a FAQ.

[DPDISXR4Ti]

Good job Luke. But man, it seems like every time the discussion comes up about the 7.5 GM swap, some new and different issue comes up. We did have a several-page thread on this topic several months back - you might want to check it, and you'll see.

BTW, unless that diff traveled in a time machine, I'm guessing it wasn't out of a 1986 anything. Note the picture below as evidence, your honor....

[MerkXRTurbo]

Brad,
Yes, I followed that thread religiously as it was active, and I referred to it a number of times during the weekend as I was attempting to figure out how to get the damn carrier into the case.

Yes, I saw that number and wondered if the 1986 recollection was incorrect, however it looks like it may the Canada patent #??? Although that's how it looks, it doesn't seem right. One would think that Canada would have had much more than 2000 assigned patents by the time the late 80s rolled around, but you never know... it is Canada.

[Frag]

I was wondering if you just fell off the face of the earth Luke.

[MerkXRTurbo]

BTW, I found pics of an exact copy of the carrier that I have. Reports from the owners say that they got them from a 1987 Trans Am.

Also, looking up the patent # I found that it was issued in late '86, so it couldn't have been a 1986 Trans Am.

[MerkXRTurbo]

Here's some pics that I found. Note that the ring gear bolt pattern on the one like mine looks slightly smaller... that is probably why my XR ring gear bolted right on without elongating the holes like many have had to.

Also it looks as if the one on the right has slightly larger axle holes that wouldn't require the stub shafts to be turned down. This is something that I can't find discussed anywhere, and I believe it's because the solid rear end guys don't have to worry about it since their axles are supported by bearings on the other end.





Also, I have confirmed that this is indeed an Auburn LSD. Sorry if that's stating the obvious to many.

[pyropete125]

i *think* Hal [fknbadfkr] did this swap years ago over on turboford.net

IIRC he had to clearance the housing and it worked with minimal mods.

I want to say you need the oddball, or not as common, gm 26 spline lsd.

pete

[janvanv]

i *think* Hal [fknbadfkr] did this swap years ago over on turboford.net

IIRC he had to clearance the housing and it worked with minimal mods.

I want to say you need the oddball, or not as common, gm 26 spline lsd.

pete

And speaking of splines, can we get a nice accurate CLEAR shot of the splines?

I seem to recall somewhat vaguely that the GM splines were 30 degree PA and the Xratty is as we all know, a 45 degree PA.

Seems to me that was the reason I didn't pursue the GM diff but that was over 20 yeras ago and materials and QC and technology has changed a lot since then so bagatelles like the splines being off doen't mean as much now.

[DPDISXR4Ti]

i *think* Hal [fknbadfkr] did this swap years ago over on turboford.net

IIRC he had to clearance the housing and it worked with minimal mods.

I want to say you need the oddball, or not as common, gm 26 spline lsd.

Indeed Hal did the GM 7.5 swap, but he wasn't the first nor the last. It just seems like there's a lot of variation in the install process based on the type of diff'. I chalk this up to:
1) 3.23 ratio and up are different from the series less than 3.23 (numerically).
2) Different manufacturers have different external dimensions (mostly just a case grinding issue)
3) There are 7.5" as well as 7.625" ring gears, which often get lumped together as one

I would hope that anyone trying this knows to get the 26-spline rather than the 28-spline unit, which I think arrived in the GM world circa 1988. Some things even a BFH can't solve.

John's comments about the angle of the splines is interesting, but I'd be surprised if they're different. The guy who originally did this swap ~10 years ago is a machinist and I'd be surprised if he tolerated such a mis-match. But it's worth investigating to know the actual answer.

[janvanv]

John's comments about the angle of the splines is interesting, but I'd be surprised if they're different. The guy who originally did this swap ~10 years ago is a machinist and I'd be surprised if he tolerated such a mis-match. But it's worth investigating to know the actual answer.

Yeah well I'm a machinist, and maybe that's why I noticed the difference, and rejected the idea. Been 20 years as I said.
Could also be that I don't want to go to effort for an install and still have only a 2 pinion diff---since I've been spoiled by having all steel 4 pinion LSDs since 1984 in my cars.
That is part of the reason i like the 8" Supra diff, the other being the cost of final drive parts.

[Chuck W]

This a swap I want to do once I get the T5 into the Scorpio. My head just spins from all the details about which unit is the proper one to fit.

I haven't heard of anyone having to turn stub-shafts down though yet.

Luke- In one of the pics, it LOOKS like the stamping says "7-5/8" (On the close-up of the GM ring gear). To me that says 7.625"... Your pic is fuzzy, so it's hard to tell, but that's what I see.

Are you SURE you have a proper unit?

[MerkXRTurbo]

Yes, I am 100% sure it's 26 spline. Upon removing the passenger side bearing race I put the XR stub axle into the diff, and it fits every bit as good as factory. Also, I compared an S10 axle with the XR stub and the splines are an exact match.

Chuck, I'll have to take another look at the ring gear and see if that's what it says. I think the photo shows that it is obviously the same size as the XR's ring gear. What's the "proper unit" when you're customizing something? 1st of all it is an LSD. The bearings are all the same size, so they'll ride in the case just fine. The XR ring gear bolts right to the carrier, so I get to save some time there. Maybe it's not the "proper unit" because the axles are too big... I'll have them turned down. Maybe it's not the proper unit because it will possibly need a thicker spacer than .090. Maybe it'll also need a spacer to get the ring gear close enough to the pinion. If so, I'll get one.

From what I have seen so far, it's going to work. Just like anything custom, it may require something slightly different than the other custom thing. Fact is, the design is close enough to factory that it will work.

[Chuck W]

I'm just saying that I have yet to hear of anyone doing this swap having to turn the axles down to get them through the bearings. Thus my asking.

There's a guy who is around TF and MS (deathbypsi) that just swapped in a GM 7.5-converted XR diff and just slid the axles in he took out of the original diff. He just swapped out the diffs on a lift in an hour or so. On the others, the fact that they had to turn down the axles doesn't stick out either. That's something I would have remembered reading as I have considered this swap for a while.

"Proper" unit refers to the fact that there is one out of the bunch that is better suited (ie less mods) to get it to work. If you want to do a bunch of new work and blaze some new ground, then so be it. I'm thinking you have the 7.625" diff instead of a 7.5" diff, which IIRC is the "proper" one.

Just trying to make sure everyone is talking about the same thing as there is enough confusion on this swap as it is without injecting more.