Gm rear diff. help

Are there any gm rear diff. guru's here, I have a strange one for you, and looking for answers.
I used a diff from a 79 caddy, so I have drove the car for a year now and want to get some more bottom end from it, just change the gears, easy, right ?, ( NO ) I pulled the cover off tonight, 10 bolt cover with the flat top and bottom and the sides are curved in on the gasket, look in the summit book and think I see the gasket as a pontiac style gasket, ok cool here we go, the carrier has 12 bolts holding the ring gear on, 7/16th with left hand threads, the carrier has 4 stamped and cast into it, so i figure a series 4 carrier, the ratio is 2.27777, so now i need a carrier to fit the new ring gear, only problem is all the 10 bolt carriers i find have 10 bolts holding the ring gear, WTF do i have, and is there anything out there to put in this thing, 3.73 is where I want to go. Any help would be appreciated thanks guys.
p.s. sorry for the spelling and pun-chu-ation, grammer was never my strong suit.

Decoder

Try this website. It has a section for de-coding GM differentials. Hope it helps.

http://www.nastyz28.com/decode.php

Lynn

sounds like you have a 8.875 diff

12 cover bolts and gasket looks like this??http://www.nationwideparts.com/differentials.htm

Sorry, thought I read 12 bolt cover.....you said 10

my bad....but the site should help you find what you're looking for...

You probably have what we call a 11 bolt in the derby world. It's a 10 bolt cover and big 12 bolt guts. It's not the same 12 bolt that everyone thinks of. They make NO parts for them. You have several option for switching the complete housing tho.

21willys said:

You probably have what we call a 11 bolt in the derby world. It's a 10 bolt cover and big 12 bolt guts. It's not the same 12 bolt that everyone thinks of. They make NO parts for them. You have several option for switching the complete housing tho.

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Seems like I'm finding that out now, I built the whole car so I guess I can rebuild it with a different diff. unless I can find an old late 70's or early 80's chevy big car rear end that will just bolt in.
I was kinda hoping I could stick a 12 bolt style ring gear in it with a spacer and make it work, thats not looking too good right now. thanks for the replies maybe someone will have a trick I can use.

2.27 is some pretty tall gears, but I bet it goes down the interstate really nice. I think the only shot you have is to change out the entire rear axle assembly for something with deeper gears. Just for giggles you might call some of the gear manufacturers like Eaton, Richmond, etc and see. Maybe, just maybe there are lower ratio gears made for that housing as it might have also been used in a heavier car or truck.

Don

2.28

yes it will 65 at 1200 rpm it just lopes around town, but it wont spin on spit. we all know thats no fun.

The only gears that you will find will be from another rearend. Not a single manufacturer makes parts for these other then bearings and seals. 77 is a funny year for caddys. It's when gm switched to a metric chassis. They are just a little bit different then other years. I am 90% sure the newer caddys are a bolt in swap. I know the 78 and newer gms are mostly the same (watch lug pattern and drive shaft yoke). Gm was bad about using what ever rearend was available that day.

If it was me I would switch to a 9". Parts are very common and cheap. I have had several metric gm chassis with 9" rearends.

Most of the B-body platform used

the same diff housings.....10 bolt car was the most popular only cad used something different that I can remember...77 on were the downsized vehicles...used "corporate" engines weren't called chevy or olds or pontiac per se, just corporate engines... so if you've mocked up the cad rear end for your build, any of the other olds, chevy pontiac diff's should be the same basic set up as for trailing arms (upper and lower control arms) whatever you want to call them......

The arms are the same or very close but the widths vary.

I was generalizing about the b-body....

21willys said:

The arms are the same or very close but the widths vary.

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You are correct about the width....did vary but mostly the differences came between the B body and the C and D bodys of the early 80's....different from the B-bodys....Oldsmobile 98 Brougham and the big Cad's were C and D body cars....I assume that is what you are speaking about....yes...no... either way, you are correct, they did vary in width.....some don't distinguish the difference between them but there are differences.. I think the c and d's were about 121 ro 122 inch WB and the B body's were 119 or so... if memory serves me.....

diff help

Thanks for all the replies, but I wonder if any of them will have a 3:73 posi diff. I would hate to buy another rear, make it fit and then have to replace everything inside it also
I was thinking maybe a caprice like an old cop car but from what I found, they only had like a 3:06 gear still looking.

you-all sure I cant take a reg. 8.5 carrier and just make it fit with a few shims

Kidding Right???

Hopefully??? Kidding????

Not going to happen!

You might measure the 71-6 full size rear from drum to drum ( wheel mount to wheel mount). I know they will bolt in with a little persistence but not sure how much wider they are. The lowest factory gear for them was a 3:73. And the flang for the drive shaft is a flat cv style. Not a typical u joint style. The 71-6 gms had the same ******* rear you have, the 8.5 10 bolt and the ******* 12 bolt that nothing fits. Some had c clip axles and some have bolt in axles. The lowest gear option was the 3:73. Some of the ******* 12 bolts did have a factory posi unit. Mostly found in the olds 98s.

Now as for the 77+ gms. They also have several differentials available. Some that I've never saw. Such as a metric axle with bolt in axles. Generally they are a variation of the 7.5 10 bolt (junk). The electras, olds 98s, and caddys are the bigger lug pattern and a few of them had a 8.5 10 bolt with c clip axles. This also goes for all of the metric station wagons. The wagon rears are a bit wider. I can't remember exactly how much.

Now the caprices and cars of it size, ( the caddys, electras, and olds are bigger) all have the 7.5 10 bolt and are the small lug pattern.

Hope that helps some one. It's all from my experience with these cars as derby cars and I'm sure there is a exception to some of it.

Wow b a stard is censored?

21willys said:

Hope that helps some one. It's all from my experience with these cars as derby cars and I'm sure there is a exception to some of it.

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I'm not the only one......

I had one of those BIA rears of the 77-79 era, they still suck

21willys said:

Wow b a stard is censored?

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Yep, and so is ****, ******, G***** ****, M ************ , and the ever popular *##*&^^***.

But sometimes you can get away with ****** ********. (But I guess not this time.)

Don

Thats funny

You ( !@#$%^&*) guys crack me up sometimes , (SGT. just kidding ) 21 thanks for the info, I also have the flat style pinion flange on that 79 rear end, I will get to digging and try and find a rear from a big 71-76 gm with a posi unit, I have no problem changing the gears, just didnit want to replace the carrier also, I think the diff. in my car is pretty stout, 10 bolt case with 12 bolt internals, but like I have found and everyone has stated here, no parts for it. On the hunt for a diff. now. just so hard to find the older stuff anymore, you got to find a private source the scrap yards here have a 10 year plan, anything older gets crushed when the prices go up.

Anybody got one for sale ? (or) do you know a guy that knows a guy that thinks he knows a guy that has one ???

Caddy used the flat flange on a few rear ends up thru the 80s. Caddys are the only one tho. And usually it's a single u joint and not the cv joint.

If you find a regular 10 bolt out of a 71-6 gm parts are easily available. They are the 8.5". Good luck