# 1966 GTO complete differential swap



## Lennox (Oct 17, 2012)

Hello to all. I am curious if any of you have swapped in a newer model posi diff into a 66/67 GTO. When I mean newer i mean 68 to 72. The reason I ask is I want to replace my 3:55 non posi diff with a posi diff with higher gears. I know that Eaton, auburn etc has a posi center section available. I have access to some late 1960's era GM cars. I have not gone out to measure any diff widths yet. I was just wondering if anyone has done this and what they used for the replacement differentail. Thank You!


----------



## Pinion head (Jan 3, 2015)

Lennox, Hello up in Canada! to answer your quetions the Pontiac 8.2 carrier and ring and pinion interchanges from '64-69. '70-72 Pontiac 8.2 10 bolt carriers and ring and pinions had larger ring gear bolt holes and 1/16" larger LH threaded ring gear bolts, and as a matched set can be used in earlier housings, or even combinations of later posi, older gears can be adapted.

Some history on the carriers used. The earliest Pontiac 8.2 Safe-T-Track carriers introduced for '64 Pontiac A bodies, all were 2 pinion cone type posi carriers. In good shape, these hold up fine in good shape in putt-a-round stock type cars with narrow stock type tires. For '66, the 4 pinion low ratio (3.36-5.57) STT carrier was introduced, as the 2 pinion 8.2 units weren't holding up well under higher torque load applications. The new 4 pinion STT cone type carrier was also avail as a service piece for warranty work on '65 models. Interestingly, Olds manual trans 442's received Pontiac 8.2 10 bolts. When ordered with low ratio gearing, the 442's recieved the 4 pinion STT units. Pontiac's 8.2 gray iron center hsg was cast with much more strength than the Buick 8.2 center housing, thus Olds needed it behind the early 400 Olds and a 4 spd.

Width... Pontiac increased the width of their division's Abody rearends for the '66 models, this same width stayed under Pontiac A body's and most GM Abodys through the '72 models. NOTE: A few of the very earliest usage '66 Pontiac A body rears have been documented to be of the narrower '64-65 width. Last, Chevrolet's Malibus/Chebelles/ Elco's all stuck with the narrower A body rearend through their '67 models.

Pontiac for '67, totally understanding the need for a stronger differential, esp with its stronger and stronger optional engines, and in an effort to reduce warranty claims, began casting its higher performance ratio 8.2 10 bolt housings out of nodular cast iron. Looking back, this was a bandaid in the overall picture, but any effort to reduce center hsg flex in the pinion area helps insure ring and pinion longevity. Have torn down dozens of blown up standard gray iron 8.2 rears that owners spent a lot of coin on hard to find at the time tight 4 pinion carriers, low ratio ring & pinions, and then blew the R&P to pieces at the strip in, for the most part, mid to low 13 sec automatic GTO's and early F85's. Picking up the pieces, and starting over, one eventually gets smarter.

More timeline history... Sometime between '67 and '68, the HD Safe-T-Track Pontiac 8.2 10 bolt rears were introduced. These included the nodular center hsg, the 4 pinion STT carrier and newly introduced HD forged axles. The HD Safe-T-Track 8.2 Pontiac in Pontiac A body's was only avail with 3.90's and 4.33 gearing. Have gone through the rebuilding of over a dozen HD STT's, not something one runs across very often.

By the '68 models, Pontiac became the only GM division using the Pontiac 8.2 10 bolt. Due to neccessity, Buick came out with their own strengthened BUICK 8.2 10 bolt for '68-70. Housing design, carrier design, ring and pinion design, axles spline pitch, all are different than the Pontiac 8.2 10 bolt. Note: the much strengthened BUICK 8.2 was used under the heavy and often very quick '70 Buick GS Stage 1's (stock 500ft lbs of torque)

Olds, also for '67, came out with their own rearend design, the Type "O" 8.4" 10 bolt. The Olds guys love to refer to them as 12 bolts, but only the smooth cover has 12 bolts, in actuality, the ring gear is an 8.4" diam 10 bolt. The type "O" Olds design was used through the '70 model year Cutlasses and 442's, and again only under Olds vehicles. Both the Buick and Olds designed rears are relatively strong in top condition, stronger than the previous gray iron 8.2 Buick and 8.2 Pontiac provided rears that were under the Buick and Olds A body's from '64-67. Desirable ratio ring and pinion sets and posi's are expensive for both, more so than parts for the Pontiac 8.2. I've rebuilt quite a few of each, and up till recently, for the BUICK version there were no aftermarket gear sets.

Like Pontiacs 8.2 10 bolt, both the Type "O" and the late BUICK 8.2 had rearend housing ends which were changed slightly for the '70 model, as GM went to tapered axle bearings in all their bolt in axle A body rears. This move coincided with the introduction of wider 70 series and 60 series tires beginning in '69 in OEM applications. GM's performance A body's and early Firebirds with sealed axle bearing rears were not designed for the increased side loading, thus the move to much more positive axle retention with a splash lubricated tapered axle bearing and external seal. The great thing about tapered axle bearings, they don't seize and chew up the axle mating surface like the preceding sealed style axle bearings. 

1970... new decade, some would say the pinnacle year of the first Musclecar Era... for '70 model Pontiac GTO's built with the 455 engine, Pontiac engineers gave up on the 8.2 Pontiac 10 bolt rear and specified a McKinnon built 12 bolt rear, complete with slightly stronger cc-clip axles and a larger 3R joint flange than was used on standard usage Malibu/Chevelle 12 bolts. The major weak spot... the Eaton posi units used in the 455 application 12 bolts is they still had the same brittle spider gears and side gears as was used in nearly every oem application chevy 12 bolt Eaton Posi. Unless the rear is going back in a Putt-A-Round car, in a build, I prefer to always replace the side gears and spiders in these factory Eaton 12 bolt carriers. Have ran across too many high 12 sec cars that have shattered the small brittle spider gears.

Buick and Olds for '71 models introduced the 8.5 A body rearend under their A body product lines. Firebirds and Camaro's for '71 also received their own version, but with c-clip axles. Designed with a longer pinion and more pinion support in the center hsg than any previous GM A body differential, the '71-72 8.5 A body rear wasaver on three different housings, and was avail with two different size tapered axle bearing axles. In repetitive abuse on the street and strip, as a GM offered rearend, very hard to beat the 8.5 A body rear both in strength, availabilty of parts, and in total cost to build. Interstingly, the same strength stock axles that found themselves behind a 6cyl Skylark were also run behind '71 Stage1 and W30 models. Buick and Olds joint effort didn't cheap out on the metalurgy of the axles.

The only knock on the first 8.5's (71's) I've ever had is all of the factory '71 usage posi units I've ever examined were sourced from Warner Motive  Just another cone type posi unit that eventually wears out and is not as easily rebuilt as a clutch type posi unit. For street/strip use, I always replace the decades old 8.5 Warner Motive units with a new HD Eaton carrier. With the '72 models, the clutch type S spring corporate 8.5 Posi units began being used. This style was used up through the '70's then sparingly in the early '80's when GM was cranking out weak engine cars, then finally the 8.5's were used up through '87 in the Buick Grand Nationals. 

In building performance A body rearends off the tapered bearing bolt-in axle 8.5 A body housing and properly rebuilt 8.5 s spring posi, it's been very common to have friends and customers consistently running stock A9 and A10 axle bearing 8.5 A body rears in 3600-3800 lb A body's down into the mid 11's on slicks. With GM discontinuing the posi clutches for the 8.5 S spring carriers nearly a dozen years ago, more and more I began building HD 8.5 A body and F body rears with Eatons 30 spline carrier combined with new Moser axles. 

Hope this helps on a rundown of GM A body rears, need more detailed info, feel free and ask.


----------



## Shake-N-Bake (Jun 16, 2015)

Pinion head said:


> ...
> 
> More timeline history... Sometime between '67 and '68, the HD Safe-T-Track Pontiac 8.2 10 bolt rears were introduced. These included the nodular center hsg, the 4 pinion STT carrier and newly introduced HD forged axles. The HD Safe-T-Track 8.2 Pontiac in Pontiac A body's was only avail with 3.90's and 4.33 gearing. Have gone through the rebuilding of over a dozen HD STT's, not something one runs across very often.
> 
> ..... need more detailed info, feel free and ask.


In your experience with the HD Safe-T-Track axles used in 68-69.....did very many of them have bent axle tubes? The ZL code (4.33) axle from my car has both axle tubes arched down away from the center housing towards the brake assemblies. Looking at the differential cover....the axle is frowning a bit. Enough to be visibly noticeable to my eye. I had to take my housing to a driveline specialty shop to get the tubes straightened. Just wondering how common it is for those higher (numerically) geared axles to flex like that? Seems like it would take an awful lot of force to bend one of those things but then again maybe not. Both axle shafts came out of the housings and were straight, so I guess they must have flexed a bit while they were in there. Was surprised how beefy the axle shafts themselves are....they do not neck down in thickness like I have seen from other cars from that time period. I presume the forged HD axle shafts are the same thickness along the entire length for added strength? Was that really needed at the time? Were folks twisting axle shafts as well as breaking pinion gears back then?


----------



## Pinion head (Jan 3, 2015)

Shake-N-Bake, Hello, have yet to run across the exact same bowed hsg tubes in a project car. Have had several dozen nodular 8.2's come through over the years that i have gone through. Have run across housing tubes with ends bent up, and a real BEAST to get pairs of c-clip axles out of. For over a decade, pulled 40-50 8.5 S spring posi's a year out of Pick-N-Pull & crusher yard rears. The majorly bowed housings I had to fight the axles out of to yank the core posis from were all out of burnup late 2nd Gen F-body's as well as one Park Ave Buick. Housings were slightly bowed up from the heat & settling of the weight of the rear of the car. 

Do have several 8.2 Pontiac gray iron roller housing/rears that I stick in roller body's & fresh bought parts cars w/o rears in order to load them on the trailer. There have been several instances where I had a partscar going up onto my old open trailer (the rear 8 foot i had built open pit so I could pull rears on the trailer). In loading a 3500-4000 lb car, quite a few times was fighting dropped tailpipes or mufflers snagging on the rear cross bars of the open pit. Every time, to solve, have ended up dragging out my big floorjack & fighting to jack the back of the partscar up by the center housing. Several years ago I cked one of those roller/housings out as had an old friend/customer that was dead set on finding a certain casting dated gray iron 8.2 housing  In preparing to ck that housing in my line-up bar, sure enough the hsg tubes were tweaked down slightly. Only thing I could figure is previously I had bottomed out both rear wheels/tires in the wheel houses as i continued jacking the center housing up to free the car from the trailer. Something I've tried not to repeat, & with my new open trailer, something that would be hard to do. Hope this helps, have never ran across such a deal from hard lauches on slicks. Glad to see you had a local shop that could straighten the housing.


----------



## Shake-N-Bake (Jun 16, 2015)

Thanks for the reply. when we bought the car in the late 80's...it had been repainted a different color (Mayfair maize), the black folding top was replaced with a white top and the engine was replaced with a 400 from a 67 Firebird. Only the black interior remained unchanged. In February of this year, I finally started working on the restoration of this car and when it came back from the media blaster we had an unpleasant surprise. The LH quarter panel looked like a crumpled up piece of paper that someone tried to flatten. There had to be 1/2" of filler in some spots....I am amazed it lasted this long without falling off in giant chunks.

It is obvious the damage to the quarter panel was a result of a serious collision. Luckily the frame is square and flat so I got lucky in that regard. One of the rear control arms is not original....it likely came from a later car because it is boxed for the sway bar so the original was likely damaged in the original accident. Perhaps the nature of the collision caused the vehicle to leave the road and perhaps the differential got high centered on something. I suppose that could explain the bent axle tubes?

At any rate, it's straight now. AZ Driveline and Differential did a fantastic job straightening the tubes and rebuilding the entire assembly. Should be good for its next life coming up in the near future. 

Out of curiosity.....any idea what I would expect to pay if I had to find a replacement axle?


----------



## Will H. (Sep 4, 2021)

Pinion head said:


> Lennox, Hello up in Canada! to answer your quetions the Pontiac 8.2 carrier and ring and pinion interchanges from '64-69. '70-72 Pontiac 8.2 10 bolt carriers and ring and pinions had larger ring gear bolt holes and 1/16" larger LH threaded ring gear bolts, and as a matched set can be used in earlier housings, or even combinations of later posi, older gears can be adapted.
> 
> Some history on the carriers used. The earliest Pontiac 8.2 Safe-T-Track carriers introduced for '64 Pontiac A bodies, all were 2 pinion cone type posi carriers. In good shape, these hold up fine in good shape in putt-a-round stock type cars with narrow stock type tires. For '66, the 4 pinion low ratio (3.36-5.57) STT carrier was introduced, as the 2 pinion 8.2 units weren't holding up well under higher torque load applications. The new 4 pinion STT cone type carrier was also avail as a service piece for warranty work on '65 models. Interestingly, Olds manual trans 442's received Pontiac 8.2 10 bolts. When ordered with low ratio gearing, the 442's recieved the 4 pinion STT units. Pontiac's 8.2 gray iron center hsg was cast with much more strength than the Buick 8.2 center housing, thus Olds needed it behind the early 400 Olds and a 4 spd.
> 
> ...


Hello from Georgia USA. I was researching axel housings and ran across your post. It appears you know a lot about gm rear ends. I have a 1966 442 with turbo 400 tranmission. The axel bearings fail repeatedly. I want to change to the later housing with tapered bearings. Can you tell me which years will fit?. This is a daily driver,, not a hot rod.. I hope you can help. Have a good day.


----------

