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Old 02-15-2008, 10:55 AM
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7.5 to 8.5 swap

On my 87 Monte Carlo SS i have the stock 7.5 10 bolt but that is coupled to a 383 and TH350. I want to upgrade it to a 8.5 10 bolt but dont have a lot of money. I know the Grand National and 442 Olds are a a direct swap, but around where i live they want $1000 plus for a non posi. However we do have plentiful amounts of 8.5's from 60's and 70's for like $200 that have posi. So i was thinking that if i could get my hands on a 8.5 posi with leaf springs, couldnt i just weld up some leaf mounts. I know the rear would be a little bit wider, but that would be better for me (I need some more space for the rims i have). So how hard would it be to just swap to leaf springs?

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Old 02-15-2008, 12:27 PM
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I would not consider putting leafs in your G-Body. You are correct about finding a 8.5 from a 83-84 HO, 85-85 442, or 84 and up T-Type or GN, pretty scarce and if you do find one it's mucho bucks.

Here's some info I found researching this same idea...

I read somewhere that the 1964 to 1967 A-Body cars (intermediates) chevelle, cutlass, gs Buick, rear end will bolt into a G-body with minor love taps to the upper trailing arms. They said it is inch wider on both sides, but other than that, it is the 8.2 inch 10 bolt. Much stronger than the 7.5in. The 1968 to 1972 A-body differential will bolt up. Summit sells a control arm set that will properly adapt them. I think these from Ebay are similar: HERE

I found this:
"Swapping a 12 bolt into a G-body is probably the most difficult conversion to complete. The problem lies with the difference in control arm mount angles. You can force the arms to bolt up, but suspension movement will bind up severely. The housings are also different widths which can be corrected with wheel offset.

The first thing that needs to be done is to fabricate a jig fixture to locate the lower control arm mounts and stock pinion location. At this point you need to remove the lower control arm mounts and spring pads from the 12 bolt housing. You also need to remove the same pieces from the 7.5 10 bolt. Next, install the 12 bolt housing in the jig and also 10 bolt control arm mounts and spring pads. After double checking your measurements, you can weld everything up. Now the lower control arms (of the 12 bolt) will be at the same angle as the 10 bolt was. The upper control arms are a different story. Since they are part of the cast center section, they cannot be modified. The solution is to use an upper control arm that has spherical rod ends to allow for the mis-alignment of the angles. At this time it would be wise to add "no-hop" bars to allow adjusting of the instant center. The control arms and no-hop bars I run are available through Art Morrison Enterprises. The rod end also allows for pinion angle adjustments, which comes in handy when you need to compensate for track changes. Being able to make such adjustments will make your car be able to hook even when track conditions change. Very important in my opinion!

NOTE: Careful measurements will need to be taken to allow for proper wheel/tire fitment. The 12 bolt housing is wider than the stock 7.5 rear. I am still looking for the stock 7.5 rear width (drum-to-drum), but will provide the 12 bolts width for your comparison. The 1964-67 Chevelle housings measure 60.5 inches drum-to-drum. The 1968-72 housing measures 2 inches wider which would be 62.5 inches drum-to-drum."

You could also buy a Ford 9" made for G-Body cars. Check out the dirt/circle track suppliers like Speedway Motors or Day, not too expensive for just the housing. Of course you'll need the chunk and axles but you could buy the pieces as you get the money.

...anyway, I don't know if I helped or just made things more confusing for you. Good luck with whatever way you decide to go.
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Old 02-15-2008, 12:51 PM
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I'm really not sure...i can imagine that it wont be simple just because the Monte Carlo was never designed to have leaf springs (to my knowledge). Not to mention you would be taking a step backwards as far as technology goes.

I have a 1984 Monte Carlo SS and was in the same situation as you, wanted to upgrade but G.N. rears were too expensive, everything else was to wide and the mounts were different. one option is, and i haven't personally researched it, but I'm told that with a conversion type upper control arm a 12 bolt or 8.5 ten bolt from a Chevelle or similar older gm car with trailing arms will work, but its going to be wider which you said was ok. I think the control arm can be found on ebay, used to be any way.

the route i took was to narrow a 9 inch, but you can find one near to the desired width you want. Remove the old rear end and use it to make a jig to locate all of the mounts on the 9 inch. Then placed the 9 inch in the jig, cut the mounts off of the old axle (except for the upper mounts that had to be remade because they are cast into axle) and weld everything up. paying careful attention to the pinion angle. this method however was not without its own set of challenges
outlined in this thread

whatever route you take it will be well worth it because that axle is a serious weak point. i used this site allot, the tech section is a good resource http://www.maliburacing.com/

tiek101 beat me to the punch, BTW, drum to drum the stock g body rear end is 57 3/4 inches, measured it many times during my conversion.

Last edited by leldai73; 02-15-2008 at 12:59 PM. Reason: beat to the punch
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Old 02-17-2008, 11:30 PM
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Southside Machine used to make a conversion control arm set that would adapt a pre-'73 A-body rear end to the G-body. I talked to someone there a couple of years before they shut down, and he said they were on the verge of introducing a set that would adapt the '73-'77 rears to the G-body, but I don't know if they ever got them into production.

I understand Jeg's somehow started selling their own version of the SSM stuff, so maybe they have something.
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Old 02-18-2008, 06:36 AM
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Quote:
Originally Posted by mightycarlo10
On my 87 Monte Carlo SS i have the stock 7.5 10 bolt but that is coupled to a 383 and TH350. I want to upgrade it to a 8.5 10 bolt but dont have a lot of money. I know the Grand National and 442 Olds are a a direct swap, but around where i live they want $1000 plus for a non posi. However we do have plentiful amounts of 8.5's from 60's and 70's for like $200 that have posi. So i was thinking that if i could get my hands on a 8.5 posi with leaf springs, couldnt i just weld up some leaf mounts. I know the rear would be a little bit wider, but that would be better for me (I need some more space for the rims i have). So how hard would it be to just swap to leaf springs?
Just so you know a 8.5 G-body rear will require you to shorten your drive shaft 1" or get the one from the donor car.
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Old 02-18-2008, 03:16 PM
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According to malibu.com a mustang 8.8 is a virtual drop in, A Ford exploder rear can work to with some welding search on malibu.com you find all the info there.
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Old 02-19-2008, 10:44 AM
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Quote:
Originally Posted by barnym17
According to malibu.com a mustang 8.8 is a virtual drop in, A Ford exploder rear can work to with some welding search on malibu.com you find all the info there.
Its definatly not a drop in. You will need to make a jig and weld on the control arm and spring and shock mounts.
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