# Sticky  12-Bolt Chevy Rear End Into A Pontiac GTO



## PontiacJim (Dec 29, 2012)

*Chevy A-body 12-bolt in an A-body Pontiac.*

Posting this info/swap as this question comes up often enough. I put together what info I could find, but there could be more that I missed. The big thing is the driver's side upper control arm has to be modified for the 12-bolt installation. Honestly, never knew of this, or anyone who modified their control arm. The 12-bolts were simply installed using the control arms found under the car.

Pontiac A-body 10-Bolt Rear Axle Width:
1964-65 - 56 1/2" backing plate-to-backing plate
1966-72 - 58 1/2" backing plate-to-backing plate

Pontiac A-body Upper Control Arms:
1964 - 12 3/4" Center-to-center but uses a smaller upper control arm bushing.
1965-67 - 12 3/4" Center-to-center.
1968-72 - 10 1/4" Center-to-center.

Pontiac A-body Lower Control Arms:
1964-72 - 22" Center-to-center.

The 10-bolt to 12-bolt conversion, or the factory 12-bolt, uses a different driver's side upper control arm than the 10-bolt, but the 10-bolt arm can be modified to work.

The driver's side upper arm will hit the 12-bolt housing if it is not modified. The factory trimmed the arm and then added a reinforcement plate to compensate for the material removed. Easy enough to duplicate.

*Pic 1 *shows how/where the 10-bolt control arm will hit the 12-bolt housing.

*Pic 2 *shows the factory reinforced 12-bolt arm at the top and the factory 10-bolt arm at the bottom.

*Pic 3 *shows a homemade template based off of a stock 12-bolt control arm having the plate. Without a 12-bolt arm to copy, I would do a fit & trim/grind process and repeat as needed until I got the clearance so the factory 10-bolt arm did not make contact with the rear end. That would give me the contour needed for the reinforcement plate and then I would make a template similar to the photos. Then cut out a reinforcement plate and weld it to the factory 10-bolt control arm to provide additional stiffness. One of the forums also suggest using a large heavy gauge washer. No thickness for the reinforcement plate is given, but I would use a 1/8" thick mild steel plate for the reinforcement piece.

Not too big of a job if you have the equipment. If not, you could have a fabrication/weld shop do this for you.


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## Lemans guy (Oct 14, 2014)

I did the 12 bolt swap and used aftermarket adjustable upper control arms. I had used the originals for a time and did not recall even seeing this difference.


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## Mine'sa66 (Oct 30, 2019)

I installed a 12 bolt into my 1966 and did not have to modify anything.
Not all 12 bolt housings are_ exactly _alike. There are minor differences.
Just throwing that out there.


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## PontiacJim (Dec 29, 2012)

Agreed. I have never known about any differences in the uppers. When swapping one rear for another, I just used the control arms that were with the car/factory, and bolted the rear in. So who knows? Just wanted to include that in the write up just in case someone did have a clearance issue and could not figure out what the problem/fix is.

The aftermarket adjustable tubular control arms wouldn't be an issue as they typically use a different style ends.


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## 67lemans (Oct 30, 2009)

Excellent info. Thanks for sharing it w pics no less!


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## Duff (Jan 12, 2020)

67lemans said:


> Excellent info. Thanks for sharing it w pics no less!


I installed a 12 bolt from a 67 SS Chevelle into my 66 GTO, no mods were necessary. Upon closer inspection, the top control arm on my 66 WAS making contact with the 12 bolt housing. I just ground the arm a little, Jim points this out out a few threads back.


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## Baaad65 (Aug 29, 2019)

I did a custom built '66 Chevelle 12 bolt into my '65 with no problems made semi boxed the upper arms, installed SSM lower lift bars and triangulation tube bars, a 1" anti sway bar with KYB gas adjust shocks...I'm happy with the setup.


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## CoveKid19 (Nov 18, 2021)

After reading this thread I thought I'd go take a peek at the 12 bolt El Camino diff in my 66. Sure enough there's very little clearance. Might even make contact if there's enough movement, so since she's grounded while waiting on the TKX I guess I'll pull the uppers and mod them a bit. These are a stamped part so I can see how there's enough difference that some will fit and some won't.


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## Baaad65 (Aug 29, 2019)

I checked mine recently too and the drivers side looks like it's touching the house so I will be fixing that while changing out the springs.


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## Baaad65 (Aug 29, 2019)

So just got a couple pictures and it's definitely touching so what try and get a grinder in there or a punch to make some space?


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## Baaad65 (Aug 29, 2019)

...probably best to try and get it out while I'm changing springs, not crazy about making a lot of sparks by the gas tank.


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## CoveKid19 (Nov 18, 2021)

Baaad65 said:


> ...probably best to try and get it out while I'm changing springs, not crazy about making a lot of sparks by the gas tank.


With the shocks disconnected the axle will drop and make plenty of room to raise the arm and get a tool in there. I thought about using my mini belt sander to do the job until I couldn't find it. I don't remember if I loaned it to someone and never got it back, or lost it in the move after I retired. Guess it's time to go buy another one.


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## Baaad65 (Aug 29, 2019)

So my question now is would I notice anything if it's just rubbing a little bit, does it need to relieved ?? Can't believe I didn't notice this when I assembled everything on the bench, the bolts slid right in and the arm moved ok.


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## Baaad65 (Aug 29, 2019)

...and another question are adjustable upper arms worth it since my pinion angle seems correct? Found some SSM arms for 172.00plus tax....but like they say while you're in there doing that....


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## CoveKid19 (Nov 18, 2021)

Baaad65 said:


> So my question now is would I notice anything if it's just rubbing a little bit, does it need to relieved?
> ...and another question are adjustable upper arms worth it since my pinion angle seems correct? Found some SSM arms for 172.00plus tax....but like they say while you're in there doing that....


Metal to metal contact is never a good thing and it doesn't take much effort to relieve the arm. You'll sleep better after you do it. As for adjustable arms, I'd only install them if the pinion angle needed correction as a result of lowering or raising the suspension.


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## PontiacJim (Dec 29, 2012)

CoveKid19 said:


> Metal to metal contact is never a good thing and it doesn't take much effort to relieve the arm. You'll sleep better after you do it. As for adjustable arms, I'd only install them if the pinion angle needed correction as a result of lowering or raising the suspension.



You can grind for the extra clearance, but you are not taking into account that this can weaken the mounting ear - and why the pictures above show the metal plate you may want to fabricate and weld to that side you do the grinding on.


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> ...and another question are adjustable upper arms worth it since my pinion angle seems correct? Found some SSM arms for 172.00plus tax....but like they say while you're in there doing that....


Here is a picture of the Spohn brand adjustable upper control arms with the "Del Sphere" joints. The stock one is at the bottom of the photo. I had to modify mine and reverse them for my use with my suspension changes. You can see the shape and their thickness - pretty heavy stuff.


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## Baaad65 (Aug 29, 2019)

Ya I'm not going to leave it, I saw these also but they're the same price as the adjustables, what about poly bushings is that necessary because those cost 30.00 more.


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## Baaad65 (Aug 29, 2019)

I modified my upper arms slightly by drilling a hole in the middle and using a steel bushing that I didn't need from my sway bar and bolting in the channel, thought it would add some ridgity and keep from folding under stress.


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## PontiacJim (Dec 29, 2012)

Baaad65 said:


> Ya I'm not going to leave it, I saw these also but they're the same price as the adjustables, what about poly bushings is that necessary because those cost 30.00 more.
> View attachment 148143


Poly is just harder and can transfer more road noise through the car (yeah, like I can really hear that with headers. LOL). You also won't get much flex so you may be putting extra stress on mounting points as the body/rear end rolls - like in corners. May even cause steering issues being too stiff. Some have stated they can "squeak", but some have a grease fitting and use a special grease so you can grease them periodically. Poly-graphite is self lubricating bushings and preferred if you can get them.

Rubber will absorb a little more shock and noise, and will have flex.

So given the choice, I would feel more comfortable with poly at one end and rubber on the other.

This is why the Del Sphere joints on the ends of the adjustables - they provide flex/movement to the control arm.


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## Baaad65 (Aug 29, 2019)

PontiacJim said:


> Poly is just harder and can transfer more road noise through the car (yeah, like I can really hear that with headers. LOL). You also won't get much flex so you may be putting extra stress on mounting points as the body/rear end rolls - like in corners. May even cause steering issues being too stiff. Some have stated they can "squeak", but some have a grease fitting and use a special grease so you can grease them periodically. Poly-graphite is self lubricating bushings and preferred if you can get them.
> 
> Rubber will absorb a little more shock and noise, and will have flex.
> 
> ...


That's what I was thinking, that's why I went with graphite body bushings...didn't want my teeth rattling.


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## CoveKid19 (Nov 18, 2021)

PontiacJim said:


> You can grind for the extra clearance, but you are not taking into account that this can weaken the mounting ear - and why the pictures above show the metal plate you may want to fabricate and weld to that side you do the grinding on.


I decided to follow your advise Jim. For added peace of mind I welded a thick washer to the arm and relieved the assembly for some extra clearance.


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## O52 (Jan 27, 2019)

From the Lever Family Racing Website






Create 12 Bolt Control Arms | Lever Family Racing







www.lever-family-racing.com


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## Baaad65 (Aug 29, 2019)

Just took care of mine with a dremel tool, it just needed a couple passes as it was barely touching.


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