Should I rebuild the 7.5 rear?

Hey guys I'm kinda sitting here trying to figure out some things on the rear for the Olds. I have the line on a 12 bolt that can go in with a bit of work though there is always the possibility of binding with that rear in a G Body if not lined up proper etc.

There is also the option of for now taking out my stock 7.5 10 bolt rear and rebuilding it so that it is a a bit beefier. This second option would actually be cheaper over all and would of course fit in proper and not have some of the issues I would have swapping to the larger (heavier) 12 bolt.

From what I understand you can fit an F body posi into the 7.5 housing then put in 28 spline axles meant for the GN. This gives you a much beefier rear end than the stock 7.5 peg leg. Benefits are its a stronger rear that could get me by until I can build up a proper Ford 9 inch (or source one) and also it would not cost me an arm and a leg to do.

The 12 bolt is of course a much stronger unit from the word go but was not designed to fit the car. It can fit with some "massaging" but to get a perfect fit it will require removing and rewelding all mounting points etc. This rear is also 2 inches wider and would throw off my wheel off set (not a huge deal but something to consider). It is also a much heavier rear, the one I am looking at is not a posi and is unknown gear ratio so I might be replacing all the guts anyway.

So what do you think? Build the 7.5 up for now and save up for the Ford 9 or the 8.5 from a GN or buy the 12 bolt and go that way? I'm really starting to lean to the rebuild the 7.5 and go from there.

Car is going to have a 300ish HP 350 and a 4spd trans. Not going to drag it, might do the occasional autocross and spirited drive on mountain roads. Other then that it will be just a cruise around fun rig.

Have you thought about the ford 8.8 instead of the pricier 12 bolt but with similar durability? They came in just about everything in the last 20 years.

I think your rebuild plan is fine.

You need to ask this question over on the W.A.R. board Weekend Auto Racer.

http://www.auto-ware.com/autoware-bin/tech.pl

Those guys know what fits what and someone might even have one ready to go in for sale.

44

I sure hope your rebuild plan is fine, that's exactly what I'm putting in the MG

I know they are heavy cars, but if you're not going to be launching it on slicks, and you're planning on around 300 horsepower...is the upgrade (to the 10 bolt) really even necessary?

I mean...if you're putting something in "to get you around" until you can swing a 9" or whatever, I'd say just throw a used posi in it (the 10 bolt) and go. If you break it before upgrade (9" or whatever) is ready, throw another one in and add a throttle stop (j/k).

My thoughts are that if you're looking to a stouter rearend anyway, don't sink a bunch of money on GN axles and such...

That's one outlook on the matter,
Clem (<--hasn't broken a rearend...nor has he had a 300 hp car)

most of the 3rd gen f body people posting on this subject on their forum say the 7.5 with a posi is good for 350 HP sustained if it is built with 28 spline axles and all the tolerances are good. so i would think a 300 HP project would be fine for you intended use. just be careful to get a unit set up for 28 spline axles. the 88 and earlier F bodies had 26 spline set ups even with posi units. the switch happened in 89 so its a mixed bag if you buy an 89 model.

Keith wrote: I sure hope your rebuild plan is fine, that's exactly what I'm putting in the MG

You are planning a 7.5 house with the posi internals from an F body?

Clem one of the reasons for the upgrade is the stock 7.5 10 bolt has weak points including the 26 spline axles. Also this is a unit that has had god knows what done to it in the past and needs a rebuild anyway before I put any sort of power in front of it.

I had not thought of the 8.8 that could be an option as well considering my local junk yard has several Mustangs laying about.

Crown vics...did they use the 8.8?

A guy got a crown vic from me for demo derby and said he bolts gm b-body rearends into them...so the interchange might not be that far off...

On the 10 bolt rebuild...I understand that...30 year old rearends...it's probably just a good idea anyway.

Clem

ClemSparks wrote: ...I understand that...30 year old rearends...it's probably just a good idea anyway. Clem

I was trying to leave out the 30 years old part...as of the 25th thats how old I'll be. Hmm wonder if I need a rebuild

rebelgtp wrote:

Keith wrote: I sure hope your rebuild plan is fine, that's exactly what I'm putting in the MG

You are planning a 7.5 house with the posi internals from an F body?

Exactly. 7.5 rear axle from an S10 blazer, filled with the Positraction diff from a 1998 Camaro and fitted with custom 28 spline Mosler axles.

I have a 7.5 posi in my SS and have never had any problems, although no it dosn't spin both wheels as well as it used to.

There is a way to put the Mustang 8.8 into a G body. The Google is probably your friend here.

Car Craft Feb 2010 issue, page 43, covers this very subject. Here's a rundown;

• 12-second G-bodies live with the 7.5" so it's doable.
• A 3.42:1 rear gear setup is probably as deep as you want to go because it has greater tooth contact area than a 4.10:1 or similar, so stick to this or the 3.73:1 at most.
• Use a solid pinion spacer instead of the stock crush sleeve, available through Summit from Ratech (PN 4111)
• Detroit Locker TruTrac (PN 912A317) for $376, requires 28 spline axles
• 28 spline Mosers (PN A102808) for $245
• Richmond 3.42:1 gears (PN 49-0007-1 from Jeg's)
• Ratech install kit (PN 3001K) for $110
• Aluminum rear support cover (PN 8510400) for $150

Do it yourself for a $800 or pay a couple hundred more to have it built by a jobber. The TruTrac sounds like an excellent diff and as long as you aren't abusive it should live a happy life under your Olds.

pres589 nice sounding setup there that beyond the way I was thinking of going with the rebuild of the 7.5. Though it might be a good way to go. I will have to see if I can find a copy of that mag.

Keith yeah that is the basic setup I had heard about (though using the G body 7.5 of course). Aside from the S10 rear do you mind me asking how much you are into it? Any other bits and pieces you added or changed to make it work?

What's the problem, too much money? Shop around and you can probably find some of this stuff in good condition used but I would hesitate to cut out much of it. Good progressive LSD that will give will act nicely on the street, good gears spinning good axles and a good rear cover that should help keep the housing a little more straight under power.

rebelgtp wrote: Keith yeah that is the basic setup I had heard about (though using the G body 7.5 of course). Aside from the S10 rear do you mind me asking how much you are into it? Any other bits and pieces you added or changed to make it work?

Since I already had a 1998 Camaro, all I've purchased has been the S10 axle and the Moser axles. They were an off-the-shelf part with custom ends, so they only charged me the price for their standard S10 axles: $245. I haven't had the Camaro diff installed in the housing yet so I don't know what that will cost. I've done a lot of custom work to install the rear in the car, but that's not really relevant.

I wasn't planning on doing anything unusual with the rest of it, the diff was holding up behind the same engine in the Camaro and I'm taking a significant amount of weight off the car. My car has a Posi rear end, 1999 and later F-bodies came with a Torsen diff that's similar to a Trutrac. Given how well the Cadillac Posi works in our Miatas, I'm actually pretty happy with that.

I don't have direct experience, but the guy I bought my Caballero from (ZZ3 crate motor, Muncie M21, Auburn Pro diff, I don't recall what axles) used the car for autox/track and said he use to go through a ring and pinion every year or two. When I bought the car it just had a new 3.73 installed. Unfortunately, the motor went before I had a chance to see if the rearend was a liability or not. Hopefully I'll get the chance this spring... .