Best axle swap

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JohnnyUtah1

Not-quite-so-new-guy
Aug 17, 2018
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Hi new to the forum, never been on a forum so let me know if I’m foruming wrong...

I have a 1987 gmc Caballero and that was “restored” by a tech school before I bought it from a neighbor It has the stock 305 and I want to know how to find out what transmission I have and what axle I have?

Right me if I’m wrong but I’m pretty sure the axle would have the 7.5 ring gear and 2.42 spider gears correct? I know it does not have a posi

Any suggestions on the cheapest way to do an axle swap to get limited slip?

What G body’s came stock with the posi 8.5 and 3.73s I think it’s a 12 bolt axle?
 
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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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I'd never tell anyone what they need to do with their ride, but I will offer some free advice on some considerations, as it were:

When approaching this subject, based on your other post, I gather your dream is to have some serious power up front, and you're wanting to beef the drive train up in order to have a strong driveline to handle all the power. Good thinking, and starting with the rear end first isn't a bad idea. Especially if you have limited funding. Nothing wakes up a 30x cubic inch anything faster than a good set of gears. IIRC,83 Hurst/Olds in the G-body genre has 7.5 rear with 3.73, and 84 Hurst/Olds and 442s had 8.5 with 3.73. Monte SS I think had a bastardized 7.625" rear gear with 3.73 (might be wrong on that) Buick GN I believe had 8.5 with 3.42 or 3.73 optional. I could be wrong, I don't follow much other than G-body Oldses.

But projects like this change the whole aura of the vehicle. Thus, you may want to consider the end goal first, and take into consideration of everything you'll need to get there. Like potential body stiffening, how you're going to stop that thing once it's tearing up asphalt (brakes), how it leaves so you can grip the asphalt (tires/suspension) and do you care more about power or fuel economy? Can't have both no matter what you do, but you can come closer with say, an LS engine with fuel injection or other concessions.

The old phrase of "how fast to you want to spend" comes into play here. Plan the work, then work the plan. Put some fudge in there so if you change your mind from a bbc and go with an LS1 or something, you can do that.

For example, if you decided your end goal was an 800 HP monster, you might want to start thinking Ford 9" modified rear end, or something built to handle that power. But if it's a modest 350 HP give or take, you can get by with an 8.5" ring gear with 3.42 or somewhere around there so you can enjoy it on the street as well as the strip. Your canvas is blank and getting started is one of the hardest things to do.

Good luck, whichever way you go.
 
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lilbowtie

Comic Book Super Hero
Jan 7, 2006
3,460
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Canton Mi
Google GM transmission ID and GM rear end ID - great learning experience. Cheapest way would be a Monte ss rear(7.625) since they are the most plentiful. You might find a 7.5 posi or you can also put a posi unit in your rear end.
 
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pontiacgp

blank
Mar 31, 2006
29,270
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Kitchener, Ontario
Whe I upgraded my GP I did the frame, brakes, suspension and then the drive train. It's not hard to make a car go fast but if it can't stop or turn then you're looking for trouble.
 
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JohnnyUtah1

Not-quite-so-new-guy
Aug 17, 2018
19
15
3
I'd never tell anyone what they need to do with their ride, but I will offer some free advice on some considerations, as it were:

When approaching this subject, based on your other post, I gather your dream is to have some serious power up front, and you're wanting to beef the drive train up in order to have a strong driveline to handle all the power. Good thinking, and starting with the rear end first isn't a bad idea. Especially if you have limited funding. Nothing wakes up a 30x cubic inch anything faster than a good set of gears. IIRC,83 Hurst/Olds in the G-body genre has 7.5 rear with 3.73, and 84 Hurst/Olds and 442s had 8.5 with 3.73. Monte SS I think had a bastardized 7.625" rear gear with 3.73 (might be wrong on that) Buick GN I believe had 8.5 with 3.42 or 3.73 optional. I could be wrong, I don't follow much other than G-body Oldses.

But projects like this change the whole aura of the vehicle. Thus, you may want to consider the end goal first, and take into consideration of everything you'll need to get there. Like potential body stiffening, how you're going to stop that thing once it's tearing up asphalt (brakes), how it leaves so you can grip the asphalt (tires/suspension) and do you care more about power or fuel economy? Can't have both no matter what you do, but you can come closer with say, an LS engine with fuel injection or other concessions.

The old phrase of "how fast to you want to spend" comes into play here. Plan the work, then work the plan. Put some fudge in there so if you change your mind from a bbc and go with an LS1 or something, you can do that.

For example, if you decided your end goal was an 800 HP monster, you might want to start thinking Ford 9" modified rear end, or something built to handle that power. But if it's a modest 350 HP give or take, you can get by with an 8.5" ring gear with 3.42 or somewhere around there so you can enjoy it on the street as well as the strip. Your canvas is blank and getting started is one of the hardest things to do.

Good luck, whichever way you go.
Whe I upgraded my GP I did the frame, brakes, suspension and then the drive train. It's not hard to make a car go fast but if it can't stop or turn then you're looking for trouble.

How did you upgrade the frame?
 

Macguyver

SOUTHSIDE MACHINE PERFORMANCE
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Mar 21, 2017
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Best option and cheapest option is something to consider. The G-body 8.5 is a rare bird these days. Came in some of the later 442's and the Grand National. They can usually be found in the $1000 range, depending on how long you search for one.

The stock 7.5" will last for a while if you avoid drag slicks and traction.

The GM 10 and 12 bolts from the A-Body (68-72 Chevelle GTO Monte Carlo Cutlass an so on) fit with a conversion kit. Those rears can be found between $300-$1000, depending on how long you search for one.

https://southsidemachineperformance...olt-swap-double-adjustable-upper-control-arms

The Ford Explorer uses an 8.8" ring gear (basically a GM 12 bolt clone) and can be found for around $100 or less, again depending on how long you search for one. This also requires a kit.

https://southsidemachineperformance.com/products/explorer-88-swap-kit

Another common solution is to have a Ford 9" built and delivered for around $1800-$2200 if you don't have a tight budget.

Another thing to consider is the poor geometry these cars came with and weak trailing arms. All of our kits are focused around these weaknesses.

All of the above solutions can offer multiple gearing choices and a positraction as well.
 
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pontiacgp

blank
Mar 31, 2006
29,270
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Kitchener, Ontario
How did you upgrade the frame?

took the body off, cleaned up the factory welds and upgraded the shity welds and voids. I used a stick cause of all the dirt in the factory welds made it impossible to mig. I also boxed the C channels with 14 guage metal. I welded some straps out of 14 guage to the supension pick up points and made some 1" diameter 14 guage wasters that I welded on the tabs for the bolts for the suspension to strengthen up those areas. I removed the rear bumper shocks and opened the anchor bolt hole for a 7/16 bolt so the hidden trailer hitch I was installing to tie up the rear rails would be better mounted. I also used upgraded bolts on the shock flange for the hitch.
 
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JohnnyUtah1

Not-quite-so-new-guy
Aug 17, 2018
19
15
3
Best option and cheapest option is something to consider. The G-body 8.5 is a rare bird these days. Came in some of the later 442's and the Grand National. They can usually be found in the $1000 range, depending on how long you search for one.

The stock 7.5" will last for a while if you avoid drag slicks and traction.

The GM 10 and 12 bolts from the A-Body (68-72 Chevelle GTO Monte Carlo Cutlass an so on) fit with a conversion kit. Those rears can be found between $300-$1000, depending on how long you search for one.

https://southsidemachineperformance...olt-swap-double-adjustable-upper-control-arms

The Ford Explorer uses an 8.8" ring gear (basically a GM 12 bolt clone) and can be found for around $100 or less, again depending on how long you search for one. This also requires a kit.

https://southsidemachineperformance.com/products/explorer-88-swap-kit

Another common solution is to have a Ford 9" built and delivered for around $1800-$2200 if you don't have a tight budget.

Another thing to consider is the poor geometry these cars came with and weak trailing arms. All of our kits are focused around these weaknesses.

All of the above solutions can offer multiple gearing choices and a positraction as well.
Best option and cheapest option is something to consider. The G-body 8.5 is a rare bird these days. Came in some of the later 442's and the Grand National. They can usually be found in the $1000 range, depending on how long you search for one.

The stock 7.5" will last for a while if you avoid drag slicks and traction.

The GM 10 and 12 bolts from the A-Body (68-72 Chevelle GTO Monte Carlo Cutlass an so on) fit with a conversion kit. Those rears can be found between $300-$1000, depending on how long you search for one.

https://southsidemachineperformance...olt-swap-double-adjustable-upper-control-arms

The Ford Explorer uses an 8.8" ring gear (basically a GM 12 bolt clone) and can be found for around $100 or less, again depending on how long you search for one. This also requires a kit.

https://southsidemachineperformance.com/products/explorer-88-swap-kit

Another common solution is to have a Ford 9" built and delivered for around $1800-$2200 if you don't have a tight budget.

Another thing to consider is the poor geometry these cars came with and weak trailing arms. All of our kits are focused around these weaknesses.

All of the above solutions can offer multiple gearing choices and a positraction as well.
I’m
took the body off, cleaned up the factory welds and upgraded the shity welds and voids. I used a stick cause of all the dirt in the factory welds made it impossible to mig. I also boxed the C channels with 14 guage metal. I welded some straps out of 14 guage to the supension pick up points and made some 1" diameter 14 guage wasters that I welded on the tabs for the bolts for the suspension to strengthen up those areas. I removed the rear bumper shocks and opened the anchor bolt hole for a 7/16 bolt so the hidden trailer hitch I was installing to tie up the rear rails would be better mounted. I also used upgraded bolts on the shock flange for the hitch.

Definitely something I will be doing once I have a garage buying a house in a couple of months

Thank you for your input
 
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JohnnyUtah1

Not-quite-so-new-guy
Aug 17, 2018
19
15
3
I’m


Definitely something I will be doing once I have a garage buying a house in a couple of months

Thank you for your input


Wasn’t sure if everyone could see this and I don’t know if I can tag people to directly reply to them

But i want to thank everyone for being so adamant with helping me with my project I’ve already learned a lot


I just found a gm 10 bolt for 200 with a 7.5 ring gear and 3.73s. Now if I read right I can now buy a kit and everything will bolt up??

I wanted to stear away from the 7.5 ring gear but do you think it would be fine for a future 350 and upgraded transmission

I really just want a street machine that I could take to the strip

Also is 3.73 a decent ratio for what I’m goin for And be compatible with what I have now?
 

DRIVEN

Geezer
Apr 25, 2009
8,062
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My suggestion would be to hold off on buying any parts yet. Build the car on paper front to back before buying anything and it will likely save you dozens of hours and thousands of dollars.
READ READ READ. There is a ton of info on this site, almost too much, but it's way easier to learn from others' successes and failures.
Don't get in a hurry.
 
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