Adding Posi

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MadDrOlds

Not-quite-so-new-guy
Mar 19, 2023
39
15
8
85 442 with the factory 8.5 3.73 rear. I understand positraction was a separate option and a separate RPO. I'm admittedly pretty ignorant on gearing innards. Could positraction (1) be added to my existing rear by swapping out a gear or something or (2) does it require all new innards in my existing housing or, gasp, (3) would it require new innards and housing.

Thanks,
 

69hurstolds

Geezer
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Jan 2, 2006
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RPO was G80. Not in any package options from GM for the G-bodies, and always had to be ordered separately. GNX was the only G-body it came standard.

If your gear set is still in good repair and not worn out, you can reuse your gear set. That would save you some $$ right there. I'd probably get new bearings and seals at MINIMUM if you're going through all that trouble, though. If you can find a used GM posi center carrier from somewhere and associated guts it'd be an easy swap. Casting number for the center section is, IIRC, 410408 for the posi carrier that can handle 3.73. It has the S spring and the flat, stacked clutch plates. (Don't get the 410409 - it's for 2.56 and numerically lower number gears. It would work, but you'd need a spacer. Not a great idea) The GM carrier is pretty much a drop in. Housing is the same for posi or non posi. Of course, you'll have to set it up properly once you do all this work. That's a whole nuther ball of wax if you're not savvy at doing a job like this.

For aftermarket stuff, I would advise buying a full installation kit from places like Ratech, Richmond, etc., so you can have all the new bearings and seals/gaskets and bolts and other junk. I'd probably buy a couple extra crush sleeves to have on hand while you're at it, just in case (GM p/n 1234726 or aftermarket equivalent). They're not all that expensive, around 3 bucks, and it's easy to over-crunch them when installing the pinion. And if you don't need them, you can always sell them. Auburn makes a $400 posi rear carrier that your factory gears will bolt to with 28 spline axles. Auburn p/n 542018. It should be good up to 450 HP easy. If you're going to go higher HP, I'd suggest getting an Eaton.

OR- you can always farm it out to someone who is good at doing rear end work.

I'm no expert. I'm just thinking out loud how I'd approach this if I didn't care about originality and having to use GM parts.
 
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MadDrOlds

Not-quite-so-new-guy
Mar 19, 2023
39
15
8
RPO was G80. Not in any package options from GM for the G-bodies, and always had to be ordered separately. GNX was the only G-body it came standard.

If your gear set is still in good repair and not worn out, you can reuse your gear set. That would save you some $$ right there. I'd probably get new bearings and seals at MINIMUM if you're going through all that trouble, though. If you can find a used GM posi center carrier from somewhere and associated guts it'd be an easy swap. Casting number for the center section is, IIRC, 410408 for the posi carrier that can handle 3.73. It has the S spring and the flat, stacked clutch plates. (Don't get the 410409 - it's for 2.56 and numerically lower number gears. It would work, but you'd need a spacer. Not a great idea) The GM carrier is pretty much a drop in. Housing is the same for posi or non posi. Of course, you'll have to set it up properly once you do all this work. That's a whole nuther ball of wax if you're not savvy at doing a job like this.

For aftermarket stuff, I would advise buying a full installation kit from places like Ratech, Richmond, etc., so you can have all the new bearings and seals/gaskets and bolts and other junk. I'd probably buy a couple extra crush sleeves to have on hand while you're at it, just in case (GM p/n 1234726 or aftermarket equivalent). They're not all that expensive, around 3 bucks, and it's easy to over-crunch them when installing the pinion. And if you don't need them, you can always sell them. Auburn makes a $400 posi rear carrier that your factory gears will bolt to with 28 spline axles. Auburn p/n 542018. It should be good up to 450 HP easy. If you're going to go higher HP, I'd suggest getting an Eaton.

OR- you can always farm it out to someone who is good at doing rear end work.

I'm no expert. I'm just thinking out loud how I'd approach this if I didn't care about originality and having to use GM parts.
Thanks. It seems you are answering my basic question that it is a (reasonably) straightforward operation to add a posi diff carrier to my existing gears and housing. I'll certainly pay someone who knows what they are doing for the operation.

Its strange to me that this was a separate RPO even in the hi po cars. My 84 Monte back in the day had it, so it must have been a pretty common option to add. I guess some people wanted to preserve the g body's natural turn-on-a-dime cornering potential. Ha ha.
 

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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Actually, in my experience, anecdotal evidence points to the contrary. I’ve seen a lot more open rears than limited slip. The ordering guy at the dealership had to be someone that knew to add it to orders for the lot or the customer needed to ensure he/she asked for that option specifically for their own ordered car.
 
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MadDrOlds

Not-quite-so-new-guy
Mar 19, 2023
39
15
8
Actually, in my experience, anecdotal evidence points to the contrary. I’ve seen a lot more open rears than limited slip. The ordering guy at the dealership had to be someone that knew to add it to orders for the lot or the customer needed to ensure he/she asked for that option specifically for their own ordered car.
It would be interesting to see how many cars had that rpo.
 

abbey castro

Royal Smart Person
Oct 31, 2015
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Harker Hts TX
G80 rears are not very common options unless the guy special ordered the car an optioned the G80 unlike F Bodies. Eaton, Auburn etc are all great stuff and readily available for your carrier. Yukon Gear has them along with the kit that has all the pieces parts to completely rebuild your carrier. If it was me and money is no issue
1. Pull the whole 3rd member out
2. Buy all the parts
3. Take the whole thing apart
3. Take it to a place and have them install everything.

I wouldn't worry about the strength of the rear. unless you is making monster torque.

Now if you have mega bucks do what I did:
Pull the 10 bolt and put in a Strange S60 along with Camaro Rear Disk setup.
 

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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
8,172
17,546
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It would be interesting to see how many cars had that rpo.
Yes it would. However, GM didn’t keep records on many of the RPOs. Model, exterior colors, engine and transmission types were the bulk of what they did track. Plus anything that would be traceable in the VIN. The biggest misnomer in the G-body builds is the number of T-top cars. GM didn’t track it. So when you hear crap like 1 of 1400 with T-tops or 1 of 1835 with posi, then you know that’s BS.
 
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abbey castro

Royal Smart Person
Oct 31, 2015
1,045
1,311
113
Harker Hts TX
Yes it would. However, GM didn’t keep records on many of the RPOs. Model, exterior colors, engine and transmission types were the bulk of what they did track. Plus anything that would be traceable in the VIN. The biggest misnomer in the G-body builds is the number of T-top cars. GM didn’t track it. So when you hear crap like 1 of 1400 with T-tops or 1 of 1835 with posi, then you know that’s BS.
I have 1 of 1 1987 SS white w/Claret interior. It is 1 of 1 since no other car has the same VIN! :)
 
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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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To clarify my last post GM did keep records of every G-body built. They just didn’t compile the records in that way. And attempts for volunteers to compile those breakdowns were all turned down by GM.
 
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MadDrOlds

Not-quite-so-new-guy
Mar 19, 2023
39
15
8
G80 rears are not very common options unless the guy special ordered the car an optioned the G80 unlike F Bodies. Eaton, Auburn etc are all great stuff and readily available for your carrier. Yukon Gear has them along with the kit that has all the pieces parts to completely rebuild your carrier. If it was me and money is no issue
1. Pull the whole 3rd member out
2. Buy all the parts
3. Take the whole thing apart
3. Take it to a place and have them install everything.

I wouldn't worry about the strength of the rear. unless you is making monster torque.

Now if you have mega bucks do what I did:
Pull the 10 bolt and put in a Strange S60 along with Camaro Rear Disk setup.
Ok, I’ll bite: why would the s60 be an improvement?
 
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