GM 7.5 posi install.........

[deathbypsi]

........is done.

What a nite! I have been putting this off for a while now and tonite my buddy wouldnt let me put it off anymore. We finally had his car off his lift and he said its tonite we install my new rear or wait till spring,so I gathered my parts and headed over!

I will give as many details as possible for the people thinking of doing the swap also.

Nite started lookin like this at 9:30pm

Then I pulled the driveshaft
Unbolted my exhaust and swung out of the way
I unbolted the axles from the stub shafts{ broke 3 T40's}
Unbolted pig from the beam
Unbolted the diff mount from the floor
Lowered out of car
removed old diff cover to pull clips off stub shafts

These clips will not be reused with the GM posi because I couldnt find anyway to get them back on with the room inside the diff.

Here is how the diff looks once it is modded to fit.



My buddy had the diff done ahead of time and all he had to do was mill off about .030 of the diff case so that it would clear the end of the pinion gear.

After diff cover was sealed and bolted back up we stuffed it back into the beam.It took a little wiggling to get everything lined up again but wasnt too big of a deal.The lift made life way better and I would never do this on the floor with stands!

Only problem I had was getting the stubs out of the old rear.I thought it would be like a FWD trans where they just pop out and didnt know about the clips inside,prolly waisted a good hour hammering , prying and searching here and MS for a answer on getting them out.

2:00am took my first test drive with my new 3.36 GM posi-fied Merk!! It took about a dozen turns to get the posi to smooth out but after that it was all good! So far I love these gears way more than the 3.64's that were in it. I will have another report once I crack it open on the e-way tomorrow

[DPDISXR4Ti]

I think this is the first attempt ever of a photo-documented GM 7.5 install. You're obviously missing a few important steps, but hey, it's a start! Here's a couple questions....

1) What GM vehicle was the donor?
2) I've always heard custom machined spacers are needed for this - where are they located?

[deathbypsi]

The diff is out of a 96 Camaro.

I know nothing about any spacers. In my defence I did not set up the posi and gears myself.My buddy was going to put this in his t66 224Mercruiser/TH350 85 Merkur and never did so it was done and sitting on a shelf and all I did was throw him 125.00 and brought it home. I will ask him later today about anything special he had to do or make for the setup.

I was assuming in my post most people would know the basics of the things I didnt cover,but please feel free to add anything anyone feels should be in here!

[DPDISXR4Ti]

I was assuming in my post most people would know the basics of the things I didnt cover

Nope, bad assumption, but that's why it's a good idea to document it here.

Just doing some quick "research", it looks like the '93 - '97 Camaro came with ratios of 2.73, 3.08, 3.23 and 3.42. Even though you don't use the GM ring&pinion for this swap, the ratio matters. Why? Unlike Ford stuff, the 3.23 and higher (numerically) differentials are different from the lower ratios. Everything I've read about this swap is that you need the differential designed for the 3.23+ ratios.

Generally speaking, it seems like V8 cars got the 2.73 and 3.08 (2-series)stuff while the V6 cars got the 3.23 and 3.42 (3-SERIES). That's good news, as the V6 stuff likely got beat on less and I imagine are more plentiful in U-Pull-it's.

You should be able to determine what ratio and type the diff is from the 3-digit identifiers on the "RPO card" attached to the inside of the glove box.
"G80" indicates a LSD of some sort
"GU5" indicates a 3.23 ratio
"GU6" indicates a 3.42 ratio

Okay, little more searching reveals some confusion.... As best I can determine, a '93 - '97 Camaro is a "4th gen" car, and everything I read indicates that it only came with 28-spline differentials. In fact, it looks like the switch to 28-spline occured in 1988, about mid-way through the "3rd gen" F-body life span. Obviously we need 26-spline, so I'm going to have to call into question that what we see above is from a '96 Camaro.

[mtnXRti]

[Maurice S]

I was assuming in my post most people would know the basics of the things I didnt cover

Nope, bad assumption, but that's why it's a good idea to document it here.

Just doing some quick "research", it looks like the '93 - '97 Camaro came with ratios of 2.73, 3.08, 3.23 and 3.42. Even though you don't use the GM ring&pinion for this swap, the ratio matters. Why? Unlike Ford stuff, the 3.23 and higher (numerically) differentials are different from the lower ratios. Everything I've read about this swap is that you need the differential designed for the 3.23+ ratios.

Generally speaking, it seems like V8 cars got the 2.73 and 3.08 (2-series)stuff while the V6 cars got the 3.23 and 3.42 (3-SERIES). That's good news, as the V6 stuff likely got beat on less and I imagine are more plentiful in U-Pull-it's.

You should be able to determine what ratio and type the diff is from the 3-digit identifiers on the "RPO card" attached to the inside of the glove box.
"G80" indicates a LSD of some sort
"GU5" indicates a 3.23 ratio
"GU6" indicates a 3.42 ratio

Okay, little more searching reveals some confusion.... As best I can determine, a '93 - '97 Camaro is a "4th gen" car, and everything I read indicates that it only came with 28-spline differentials. In fact, it looks like the switch to 28-spline occured in 1988, about mid-way through the "3rd gen" F-body life span. Obviously we need 26-spline, so I'm going to have to call into question that what we see above is from a '96 Camaro.

Dolt, need a step by step w/instructions...

thanks in advance...

Homer simpson..aka...Mr. Bolt On

[DPDISXR4Ti]

Dolt, need a step by step w/instructions...

thanks in advance...

Homer simpson..aka...Mr. Bolt On

Here you go Homer....

1) Find someone who has already retreived the correct GM diff and has the required knowledge, skills, and tools to properly install it in his/your spare XR diff case.
2) Purchase said unit.
3) Remove your stock diff.
4) Bolt on new unit.
5) Done.

See, that was simple!

[deathbypsi]

Well I talked to my friend today and he said there are no spacers or anything special involved other than the milling. Its just like setting up a Ford 9" rear only different and smaller

I asked him again about where the posi came from and he said he was told it was in a 96 Camaro but it was a swap meet buy so who knows if the seller even knew,I think my friend may have just got lucky and ended up with the correct series diff. He was also told it was a Auburn but didnt bank on it since the seller was only asking 75.00 for it.

Would they have even had posi diffs in v6 cars?

[DPDISXR4Ti]

Well I talked to my friend today and he said there are no spacers or anything special involved other than the milling.

That's odd. I wonder if perhaps you've stumbled onto something that works without the spacers? For all we know, maybe it's a series 2 diff, and that doesn't require the spacers? Now that I think about it, I also seem to recall that there were some holes that needed to be opened up a bit, but it doesn't sound like you did that either?

I asked him again about where the posi came from and he said he was told it was in a 96 Camaro but it was a swap meet buy so who knows if the seller even knew,I think my friend may have just got lucky and ended up with the correct series diff. He was also told it was a Auburn but didnt bank on it since the seller was only asking 75.00 for it.

Well, it sounds like he got sold a "bill of goods" on multiple levels, cause that doesn't look like an Auburn to me either. But perhaps it happened to be a bill of goods that nobody has ever tried to install before in a Euro Ford housing, and it just happens to work better/easier than the others that have been done.

Would they have even had posi diffs in v6 cars?

Absolutely. When I was looking into this swap years ago, I recall that 2.73 ratio Posi differentials were showing up on eBay, and most often, they were pulled from rather generic GM V6 sedans. Inevitably, they were from rust-belt states.

[deathbypsi]

I was just on Auburn Gears site and their Positraction diffs look the same as mine, but I have never seen a stock GM unit either.

[DPDISXR4Ti]

I was just on Auburn Gears site and their Positraction diffs look the same as mine, but I have never seen a stock GM unit either.

I was thinking more of their cone-style units, which is what they're known for. Whether it's an Auburn or stock GM Eaton is somewhat of a moot point though, as either would have the same configuration for a given application. That's what we need to figure out - what application have you really got here? Is it a series 2 or series 3? Is it a 7.5 or 7.625?

[deathbypsi]

I was just on Auburn Gears site and their Positraction diffs look the same as mine, but I have never seen a stock GM unit either.

I was thinking more of their cone-style units, which is what they're known for. Whether it's an Auburn or stock GM Eaton is somewhat of a moot point though, as either would have the same configuration for a given application. That's what we need to figure out - what application have you really got here? Is it a series 2 or series 3? Is it a 7.5 or 7.625?

I am kicking myself in the ass now for not lookin for some part numbers on that thing before I bolted the cover back on......Honestly before I started the swap I had no idea there were so many variables in GM diffs.I thought it would be like a Ford 8.8, 28 spline and 31 spline t-lok and nothing else to think about.

Is it possible for me to just unbolt the diff mount from the floor and then unbolt the cover so it will drop down? If I can get a look at it again without having to drop the whole diff I will so I can look for some numbers or something to get some hard facts here.

[EVIL_XR4]

My GM buddy came over a minute ago and was talking about his rear end in his truck and i kinda laughed and showed him this thread. He says you need to get a GM 12 bolt or a Ford 9 inch and put it in there

I calmly explained to him that it's just not that easy.

[DPDISXR4Ti]

My GM buddy came over a minute ago and was talking about his rear end in his truck and i kinda laughed and showed him this thread. He says you need to get a GM 12 bolt or a Ford 9 inch and put it in there

I calmly explained to him that it's just not that easy.

I'm betting if you told him, "It's IRS", he'd say, "I don't care if you paid your taxes, you need a big honking 12-bolt in there!"

[mtnXRti]