'70 gto 12 bolt

Chris S

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I go soon (maybe tonight) to check out a 12 bolt rear axle out of a '70 GTO  Supposed to be a 3.73 posi.

He also might have a 8.5" from a Cutlass (not sure what year)

This would be for the '81 Grand Prix w/'76 500 strip/street car.  Any pointers on what to watch out for?

Thanks

DV8
 

Chris S

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Not to worry - he said that all those come with it at no extra charge.

DV8
 

robocop

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What vehicle(s) do you drive?
1987 Cutlass Supreme 2 door with 500 Cad/Turbo 40
Pontiac never put 3.73 12 bolts under '70 GTOs.  They had 3.07, 3.23, 3.55, 3.90 and 4.33 dealer installed.  A twelve bolt is a twelve bolt but don't let the seller bamboozle you.  If you can see the car and its an a/c car he's definitely fooling with you.  Pontiac put nothing lower than 3.07 in a/c GTOs in '70.  Neither that rear or the Cutlass rear goes easily into a G body.  You'll have to get adapter control arms or move the brackets and spring perches.  Its easier and stronger  to get a used GN or 442 rear or a converted Ford 9 inch.  The Ford's the strongest of all. 
 

Chris S

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I talked to him today and he said it may not be a 3.73 - he was guestimating as he has never had it apart.  He did say it was from a '70 GTO but the car is long gone.  This is just a rearend by itself - that's why I am asking for pointers as what to look for.

I was under the impression that an 8.5 from an '64 to '72 A-body would bolt in - why wouldn't a 12 bolt from the same car?

I'm gettin' cornfused...

DV8
 

brian

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DV8

The only bolt in is 8.5 out of GN/regal/442 that is the right width. Others either are wider or need brackets cut off and moved or both. I put a 8.5 into a 78 G-body malibu that was out of a 77 cutlass (non g-body) I had to move the lower brackets inward and shorten the upper arms to make the bolt onto the farther apart upper mounts. Also it was a couple inches wider but if you buy wheels with more inset this sin't noticeable.  The moser 9 inch I think is actually a pretty good deal for a bolt in hassle free point of view.
Brian
 

Chris S

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I can live with wider - I even already have some 4.5 & 5.5 offset wheels to work with.

You had a '77 cutlass that wasn't a G-body? :boggled:

From my research eary A and G bodies all used the same frame, the only difference being behind the axle hump under the trunk.  What would they gain by changing rear axle mounting locations?

Dv8
 

dave brode

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DV8,

Pontiac *may've* used 12 bolt chev rears in some cars, so some experts say. There is also another 12 bolt that's reffered to as an Old/Buick rear. It'll have no c-clips. If it's one of those, unless you get it dirt cheap, I'd pass on it. Hard to find parts for them, and not as stout as a chev rear, afaik. There is also another odd ball rear with a 12 bolt cover and 10 ring gear bolts.


link=topic=1068.msg10844#msg10844 date=1112196690]
I go soon (maybe tonight) to check out a 12 bolt rear axle out of a '70 GTO  Supposed to be a 3.73 posi.

He also might have a 8.5" from a Cutlass (not sure what year)

This would be for the '81 Grand Prix w/'76 500 strip/street car.   Any pointers on what to watch out for?

Thanks

DV8
 

dave brode

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DV8,

< You had a '77 cutlass that wasn't a G-body?

'64-77 Cutlass, etc was called "A". So was '78-? Not sure when the mid size went to the "G" code. Maybe '83? It's too cornfusing to me. I just say "midsized GM".
Dave


:boggled:

From my research eary A and G bodies all used the same frame, the only difference being behind the axle hump under the trunk.  What would they gain by changing rear axle mounting locations?

Dv8


 

robocop

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What vehicle(s) do you drive?
1987 Cutlass Supreme 2 door with 500 Cad/Turbo 40
Like we said the 12 bolt does not go directly into a G body.&nbsp; Spring perches are different as are control arm mounting locations.&nbsp; Southside Machine made a control arm conversion kit that allowed it to be mounted in a G body or you can cut off and reweld the mounting brackets.&nbsp; I'm not sure the control arms are still availale.&nbsp;
I was disappointed at the $400 'posi' GN rear I bought.&nbsp; It wasn't a posi, the ring and pinion were hammered and it needed a complete rebuilding.&nbsp; By the time I was done I'd spent more than if I'd bought a Moser 9 inch.&nbsp; The Olds 8.5 inch rear has a 12 bolt cover but 10 bolt innards.&nbsp; Some have a c clip axle retainer (prohibited at some drag strips).&nbsp; Isn't any stronger than a regular 10 bolt and harder to get parts for.&nbsp; Pontiac did put 12 bolt Chevy rears in '70 GTOs that had 455s.&nbsp; You'll have to look up the code or look in a '70 service manual.&nbsp; Its usually stamped on the leading edge of the right axle tube.&nbsp;
 

Chris S

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So I should be able to pull the cover, count the ring gear bolts, and check for c-clips or retainer plates.

Then if it passes muster, I will be looking at mounting it.

The lower mounts will need to be moved inboard, and the uppers will need to be shorter.&nbsp;

Also need 4-5" backspacing on the wheels.

Anything I missed?

DV8
 

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Chris, it will bolt in. You will need 5.5 BS wheels. Unless my 80 Lemans is completely "differrent G-body" and unless my buddy's 78' Malibu that goes 1.37 60 ft on 9" tires with one is also entirely different, then it will work. Everyone told me that it would not work; that it was too wide (which it is, but that can be fixed by using the 5.5" back- spaced wheels.) I put the rear of the car on jack stands and put a "floor-jack" under the differential housing and removed the 7.5 and within aa hour and a half had the other back in.
The only "tricks" I can remember are: 1) driveshaft becomes tight(looses) some of it's clearance.&nbsp; 2) emergency brake cables is tight; but it will reach back. Will have to "spread" the little clips that hold it in the hole of&nbsp; the backing plates.&nbsp; 3)the G-body cars uses metric bolts to fasten the rear in; they will work back fine. They are slightly smaller then the 1/2" ones that the earlier A-bodies use. If you want to "upgrade" to grade 8, 1/2" bolts; then you'll need to drill out the mounting brackets in the G-body's frame. 4) Shock mounting bolt on the pass. side gets close to the inside of the wheel when using the wider aftermarket wheels; just remove the 1" spacer between the the shock and the mounting bracket on the rear end housing.&nbsp; 5)&nbsp; you will have to notch the frame rails behind the rear end when using 10.5" tires that are 28 inches or taller. It's real easy to see where; have seen some guys run a 1/4 spacer on the driver side to push the tire away from the front part of the frame rail. But that is with "tall sidewall" tires; my 29.5 X 9.00's work fine.
I don't sell rear-ends for G-body conversions. I have nothing to gain by telling you this. I'm only trying to tell you what works for me and the guys I know who have G-bodies. These cars are the easiest to work on and don't have to be that expensive. Our 68-72 A body rear-ends have all bolted in. No relocating control arms or anything major; just the things I mentioned.
Hope this helps, Cooke.
 

dave brode

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robocop,


< The Olds 8.5 inch rear has a 12 bolt cover but 10 bolt innards.  Some have a c clip axle retainer (prohibited at some drag strips). 

You may be correct in that the 12/10 olds rear is 8.5'", but there's also a 12/12 olds/pontiac rear. Olds also used the corporate 10/10 8.5", from '71 up, iirc, both with and without c-clips, but the ring gears from other corporate 8.5s fit the no c-clip 8.5s. [10/10s that is].

I've often wondered why GM used so damned many different rears. Kinda like ford on their bellhousing bolt patterns.
Dave
 

Vern

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Cooke,

You say a 29.5 tall 9.0 wide fits but a 28 or taller 10.5 needs the frame notched.&nbsp; Do you think the narrower stiffer side wall of a drag radial would fit at 29.5 tall with a 10.0 treadwidth and 12.4 sidewall width without frame notching?&nbsp; This would be on 8.0 rim and the sidewall width is measured on a 10.0 rim.
 

Ted in Olympia

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I think we should take bets as to what this rear is;

I say it's a 10 bolt, 3.08 open rear.

I looked at lots of used rears for sale and they are almost always not as advertised.&nbsp; They were always low gears (most were not) and posi (almost never were).&nbsp; The only two I can remember that were as advertise was a punk'in for a 55 chevy (4.55 posi, very fun and $75) and a Dana 60 out of a Superbird (3.54 posi that I never found a use for).&nbsp; Had lots of cars that the dana would have fit but none of them seem to deserve it.&nbsp; Ended up selling it for what I had into it $100.

The good old days (late 70's, early 80's), lots of cheap fast cars.&nbsp; Now days the used parts are so expensive it's better just to build your own rear as suggested before.

TED
 

dave brode

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Ted,

<snip

The only two I can remember that were as advertise was a punk'in for a 55 chevy (4.55 posi, very fun and $75) and a Dana 60 out of a Superbird (3.54 posi that I never found a use for).  Had lots of cars that the dana would have fit but none of them seem to deserve it.  Ended up selling it for what I had into it $100.

The good old days (late 70's, early 80's), lots of cheap fast cars.  Now days the used parts are so expensive it's better just to build your own rear as suggested before.

Reminds me of two;
'68-'70 10 bolt posi olds/pontiac rear with factory 4.33s, bought for $75, blew my engine up and sold it for same.
12 bolt posi Camaro rear with 3.31s for $100. Thought that I'd make money on it. Ended up selling for $100 after advertising in newspaper.

Those were the days.
Dave

TED
 

cadiwumpas

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&nbsp; Let a 69 COPO camaro axle go with a 67 chevy II roller for a couple of hundred. Guy that later bought
the camaro put a $2,000 bounty on it . Never did track it down but I did nick him $2,000 for a pile of
427 parts...
 

cooke

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Vern, what I was trying to say, was that basically if your going to run a 10.5 tire that is 28" or taller; then the frame wells will have to be "notched" in the "curved area" of the frame in the back section . I have been TOLD ( I can not personally guarantee); that a 28 X 9.00 requires no surgery. Never tried that.
What I did try was a 28 X 10.5 MT E.T. Drag; while it was shorter than a 29" tall tire; it's added width required minor notching in the frame rails. Something else I tried and what I am currently using is a 29.5 X 9.00 E.T. Drag. It also rquires the frame to be notched in the "curve, but a little further out board. The part where I said the my 29.5 X 9.00's worked fine; I was talking about the clearance on the "front" side of the rear-end on the frame rails. When you crawl underneath the car(in front of the tire) and look up between the outer edge of the tire (sidewall's widest point) it gets close to the frame rail. My current set-up has adequate clearance as did the the 10.5's. But; that being said the 10.5's were definitely closer.That's where I mentioned some guys have had toi use 1/4" spacers. I don't.
As for your question on the "Drag" radials? I really couldn't say for sure, you'd just have to try it. I honestly don't know if all G-body cars have the same shaped wheel wells and inner contours. I know the outsides differ from say, a Malibu to a Lemans. This rear swap is something that I knew some friends had done and I was very skeptical myself; because I had always read in the "Hot Rod" books that this swap didn't fit. Out of curiousity one day; a friend and I decided to try just to see for ourselves. And it worked. Just trying to help Chris save a few bucks. I've learned not to take all the magazines at their word especially when they're trying to sell a $2200.00 rear-end.
 

dave brode

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All,
More fyi - a buddy of mine has an '87 Cutlass. He used a '65-'67 chevelle 12 bolt. He used adjustable upper arms, but he said that it bolted in. Don't know if the top arms were a must, or if he just wanted them. He did no "mods" to the rear or the lower arms. Can't say how tire clearance is except that it has olds ralley wheels and big radials and nothing rubs.
Dave
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