# Need advice regarding new differential



## Mine'sa66 (Oct 30, 2019)

Hi everybody,
This is my first post.
Always wanted a 65-67 GTO.
After decades, one finally came my way.
Now that I have it, it's now time to start working through the issues. I bought the car 180 miles from home and drove it back. It drives pretty darn well for a 53 year old car! But, it sure has a death shake over 60mph. I attributed this to the 20 year old tires in concert with the classic Cragar SS's that have the oval lug centering washers.
Well, now that I've had it a few weeks, I'm figuring more out. The tires are bad. However, I'm pretty sure the biggest issue I have is pinion angle. Right now from running the numbers, it has an open 1970 Olds rear end. This rear end has the dome spring perches instead of the correct flat ones with the keepers. I have a pinion angle finder on it's way, but knowing the deal with the spring perches and just looking at it, the differential is slightly nose down which I'm certain is no good.
So here's my question;
I want to change the rearend anyway. I'm planning on buying a complete drop in. Of course when I do so, I'll get it with disc brakes. Problem is, I've spent my fun money on the car itself, so no big moves for a while. When I get the rear with discs, I won't be able to use the single chamber master w/o proportioning valve, so that means a whole front brake kit too and 14" wheels aren't easy on rear disc so it's a snowball of money.
So the plan is to get some adjustable upper control arms to get the pinion angle ok enough to drive around until I can save up for the big upgrade.
Question is;
Since there are far more 67-up rearends out there with the domed spring perches, what's the down side to getting one of them and correcting the pinion angle with the control arms? Would it work out to be the same result or is correcting it with the arms a bandaide only?
The newer styles are usually 1" wider, which I think I might like the look of as well as being more readily available/affordable.
Thanks for reading all of that, please let me know where my thinking has gone off the rails.


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## PontiacJim (Dec 29, 2012)

So what's wrong with the Olds rear end? Probably has a 12-bolt round back cover? Stronger than a 10-bolt. It is the 8.5 rear versus the 8.2 10-bolt. Swapping back to a 10-bolt will be a waste of money since you don't plan to go original anyway.

Make sure the lower control arm bushings are all good. Then get the adjustble upper control arms if you feel the pinion angle is too much. You want a little down angle.


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## Mine'sa66 (Oct 30, 2019)

The Olds rearend is an open 2.56. I could put a posi unit in it, change out the gears, rebuild it, then buy a brake kit for it. But, by the time I do that, I might as well just purchase a complete drop in. 
I'd much rather get a nice 12 bolt for it, but boy they are spendy! I'm also moving away from my ultimate dream of a _really_ massive big block, (750+HP) and realizing a nice 450ish HP Pontiac is likely in my future, so I'm considering saving the cash and compromising with the 10-bolt.
So you do not think that using the control arms too push the pinion into correct angle against the spring perches pushing the other way is a "bandaide" fix?
Thanks to all for any input!


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## PontiacJim (Dec 29, 2012)

Mine'sa66 said:


> The Olds rearend is an open 2.56. I could put a posi unit in it, change out the gears, rebuild it, then buy a brake kit for it. But, by the time I do that, I might as well just purchase a complete drop in.
> I'd much rather get a nice 12 bolt for it, but boy they are spendy! I'm also moving away from my ultimate dream of a _really_ massive big block, (750+HP) and realizing a nice 450ish HP Pontiac is likely in my future, so I'm considering saving the cash and compromising with the 10-bolt.
> So you do not think that using the control arms too push the pinion into correct angle against the spring perches pushing the other way is a "bandaide" fix?
> Thanks to all for any input!



Any rear end you purchase will be "spendy." The new Ford 9" cost me $2,800 to set-up/build the way I wanted and that is with 11" drum brakes. A new 12-bolt won't be cheap either. The Ford 9" in my opinion is the better choice because you can more easily swap out gearing by removing the center section complete. If you really wanted, you could have another center section/gears on hand and swap them out, ie local driving gearing versus long distance highway gearing. But, another investment for another center section. But still, easier to bring the center section to a local guy to have another gear set installed and set-up. A 12-bolt, you will have to drop the car off or bring the entire rear axle assembly to him.

Purchasing another 10-bolt and beefing it up to handle additional HP & TQ will be throwing good money onto bad. You seem to be on a budget, so why throw money away and have to repeat? I understand the "get by" thinking and have done it many, many times.

With 2.56 gearing, I take it is an automatic? 1966, if factory, is the 2-speed Super Turbine. Swapping out gearing is an option, but it appears you will most likely need a different carrier. I am no expert on the Olds rear, but I found this forum that can give you more info. I don't see rebuilding the rear axle assembly worth the higher expense required to do so and in the end, may not have a strong enough rear end: https://classicoldsmobile.com/forums/drivetrain-differentials-6/70-o-type-12-bolt-rear-end-87060/

The 8.5" from a 1971-72 Buick A-body can be built to handle the HP like any 12-bolt. Many racers use the 8.5" rear end, but you may find it hard to find an A-body 8.5" that'll bolt up. Check your local craigslist as they do pop up.

So, in my opinion, anyway you look at it, it will not be inexpensive to upgrade either the Olds 10-bolt or go with a new 12-bolt and add your accessories. The least inexpensive route might be to purchase a used 12-bolt with the gear ratio you want and then upgrade from there, but again, not going to be cheap

On the question of the upper control arms, I did not say the adjustables won't work. They should work. Your springs should not be forcing the rear-end in any direction. The springs should only sit on the spring perchs. The 1966 and 67 frames are the same, so the clamp plates or towers will work either way. If the springs appear to be angled in some way, you have other issues. There are different length control arms and perhaps wrong length control arms have been installed IF they are not the original ones. It is the lower and upper control arms that **** the pinion down/up. The Chevy websites say the lowers are all the same, but the uppers are not. In 1968 the frame is changed and the uppers control arms are shorter - the frames would be about the same as Pontiac A-bodies, as well as the Olds A-body. The 1964-67 are said to be 12 3/4" center hole to center hole at the bushing, and 10 1/4" on the 1968-72 upper control arms. So IF the 1970 Olds swap included the control arms, the upper arm length may be incorrect for your '66.

But, if bushings are worn out on the lowers, they can have an effect on the pinion angle and installing the adjustable uppers may not give you what you want if the lowers are shot and need replacement. But the adjustable upper arms can dial in the pinion angle.

And, nothing on a Pontiac is inexpensive and the bigger the HP/TQ, the deeper your pockets need to be. :thumbsup:


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## Mine'sa66 (Oct 30, 2019)

I understand the issues with the Olds rearend. That's why it'll wind up elsewhere. I'm not talking about a BOP 10 bolt rearend, I'm talking a Chevy 10 bolt. One can be had for under $2000 all redone with the correct perches installed. New build or rebuilt 12 bolts are $3k and up in my shopping so far.
Which is what led me here with the control arm question. I've been speaking with a couple rearend builders who are very specific that the spring perches are an issue. 64-very early 67 will not work in late 67+up A bodies and vice-versa. I've measured my pinion angle and as I suspected. Nose down and being pushed there by the spring perches. I've asked this question of the rearend guys and their position is that I shouldn't be using the control arms to hold the angle correct in opposition to the springs. Hard launches, quick deceleration, anything that's going to want to move things around is going to be more difficult if I have things working in opposition. That seems to be pretty informed logic and makes much sense to me as a 35 year auto technician. I was hoping to come here and maybe hear from some folks who've done it.
This vehicle has factory style control arms in good condition. The perches are at a different angle and mounted higher than the correct flat ones with keepers. This is causing the nose down configuration on the pinion. Seems like if my welding skills quickly improve  I could simply cut off/weld on what I need. Doesn't make sense though on a new build rear to be cutting it up, so I guess I'll find the early model.
I will be getting the control arms anyway....then I can more comfortably drive it while saving/shopping for new parts. Also, they'll be adjustable going forward if some tweaking is needed.
Lot's of places want to sell me a Ford 9 inch. I just can't put a Ford axle under my GTO. 
Thanks for your help and I'll report back how it works out so maybe my experience can help someone else out.


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## PontiacJim (Dec 29, 2012)

A Chevy 10-bolt is not any stronger than the Pontiac 10-bolt. I get you just want to go with a GM, not Ford. The 10-bolt will work and will survive better with an automatic. I never blew one up and I don't baby my cars, but that was with a stock engine, nothing big on HP numbers. And I never had a posi 10-bolt either, so the tires would break loose before the rear would break. But, I knew of 2 people back in my time that had issues - 1 blew a spider gear through the bottom of the case and 1 twisted the axle splines.

Here is a little info on the rear ends: https://www.gtoforum.com/f39/1966-gto-complete-differential-swap-100321/

I still am baffled as to why the Olds won't work. Maybe because it is an Olds. You should have no problem installing a Pontiac or Chevy 10 or 12-bolt from a later year into your chassis. The swap is common, regardless of spring perches on the axle. The "hat" perches are 3/4" taller than the flat plate style. The base of the springs are different due to the change in perches. Perhaps the 1968 and up springs with the smaller pigtail would center better over the "hat" style perch and solve the issue. The 1968-72 rear end is 1" wider than the '64-'67 rear ends.

Check your engine casting number. If it is the 500557 block, 1975-1979, it is a weaker block due to weight saving thinner casting and not the best candidate for high horsepower if you go that route. The better blocks of that year range are the Trans-Am blocks. Pontiac re-cast the earlier 481988 and it is ID'd having "XX" cast on it.


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## Mine'sa66 (Oct 30, 2019)

Yessir, it's a crappy 500577 casting, XY code. I figured I'd end up with something entirely new, rather than building this one. I've owned quite a few 10 bolt Chevy Posi's and a couple with pretty stout engines. I've broken Ujoints and driveshafts, but not blown one of them up either. I'm still lusting after a 12 bolt though  One of the reasons I bought this car. Looks nice, drives nice, real GTO, but not much original (the interior is pretty sharp). That way I can stuff whatever I want under the hood, drive the heck out of it, and not worry about messing up original stuff. I suppose I'll get my pinion angle sorted out and just drive her. Something will probably come my way to make up my mind.
Very much appreciate the input and insight.


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## Montreux (Mar 8, 2009)

It’s always a challenge to decide how to allocate resources on an old car. 

First—Buy some tires! 20 year old tires are a hazard. They won’t quietly go flat in the middle of the night. The tread can separate, and you will have a six foot long strip of steel and rubber flogging away at your fender. Not gently unraveling, but exploding. 

Second—I’m not real clear on why you want to replace the rear axle. Isn’t the ‘70 axle the stronger 8.5” rather than the 64-67 8.2”? 2.56 is certainly not a “go fast” ratio, but it still moves the car down the road, right?

Third—Springs. ‘64 springs were large diameter at the top and pigtailed at the bottom. ‘67’s were pigtailed top and bottom. ‘64’s used a clamp plate to clamp the spring to the axle. ‘67’s did not. Unless you’re doing Dukes of Hazard jumps, the spring doesn’t need to be clamped in place! 

The raised button on the perch fits into the pigtail and locates the spring. Usually, the shocks are short enough so that even at full droop the spring cannot come loose. I think the pigtail diameter was the same from 64 to 77, so that shouldn’t be a problem. 

Fourth—Control arms. The axle is located by the control arms only. The spring only affects the height of the car. Coil spring, air bag, coil-over—doesn’t matter!

Jim is right that the pinion angle should be slightly down. Check your shop manual for the numbers. The angle of the transmission also affects the required pinion angle. 

Adjustable control arms are the only way to change the pinion angle. 

If the distance between the mounts on the axle is different between the ‘66 and the ‘70, this will change the angle between the upper arms. You will need arms that use spherical bearings rather than bushings. 

And, of course, check the condition of the bushings in the lower arms. An upgrade to urethane is not a bad idea. And boxed lower arms to allow a sway bar.


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## NOS Only (Nov 14, 2017)

Montreux said:


> It’s always a challenge to decide how to allocate resources on an old car.
> 
> First—Buy some tires! 20 year old tires are a hazard........


These statements always make me laugh. It's like tires are like that reel to reel tape deck on Mission Impossible. These tires will self-destruct in *X* amount of time.

There are many factors to tires going bad and time is not always one of them.

I have had many low mile old cars with original tires as well as bought and used old NOS tires with no problems other than the ones characteristic of that particular tire of that era. My 1957 Buick Special Riviera with 26,000 original miles ran just fine on its original BF Goodrich Silvertown wide whitewalls as does my 1988 Pontiac Fiero GT on its Goodyear Eagle GT+4's I installed in 1991. My last 1965 GTO ran on NOS Goodyear Speedway redlines dated 1972.

UV light, direct sunlight and extreme temperature changes are more detrimental to a tire than time. If a car or a tire is stored properly tires can and will last as well as function as they should for a long time, perhaps until bald.


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## PontiacJim (Dec 29, 2012)

Montreux said:


> It’s always a challenge to decide how to allocate resources on an old car.
> 
> First—Buy some tires! 20 year old tires are a hazard. They won’t quietly go flat in the middle of the night. The tread can separate, and you will have a six foot long strip of steel and rubber flogging away at your fender. Not gently unraveling, but exploding.
> 
> ...


The Olds rear is the 8.5 which is stronger, but that wasn't getting across to well. All the A-body rear ends have the spring perches welded in front of the axle tube and for some reason, it is believed that the springs should mount on top of the axle tubes and because they don't, it was applying downward pressure and the cause for the nose down pinion angle. Maybe I should have suggested using air shocks and jacking the crap out of them to counter the effects of the spring perches since the shocks are located on the rear and back of the axle tube. As you stated, the control arms are used to adjust the pinion angle, but that didn't seem to get across either. But, I don't have 35 years of experience as an auto tech nor am I an expert rear end specialist either, I am just an auto enthusiast and back yard mechanic of 45 years who has done a few rear axle swaps which I guess does not account for too much. I am still looking for a GM A-body rear axle of the 1964-72 vintage that uses spring perches set on top of the axle tube, but have yet to find an example to post. Even the Ford 9" I am using that is set-up for a GM A-body has the spring perches welded in the same place as all A-body rear axle perches.

They do make a weld on coil spring perch that could be fitted on top of the axle tube, but that would definitely change all your control arm dimensions, and you would have to alter the driveshaft accordingly. Photo included of this type perch.

So who knows. :banghead:


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## NOS Only (Nov 14, 2017)

Anyone who states that a certain rear axle assembly cannot or will not work in a certain type of vehicle should watch one of the off-road shows on TV.

Guys like Ian Johnson will take an axle assembly from a junkyard out of who knows what, cut off all of its brackets and mounting tabs to start with a clean slate, then fab up whatever they need to mount it into whatever they want.

People need to remember that an axle assembly starts out as a differential housing, 2 bare tubes and 2 flanges. Everything else is made or designed to fit.


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## Mine'sa66 (Oct 30, 2019)

Hi,
You were getting across just fine.
I'm unclear where I stated that the spring perches should be on top of the tube. My spring perches were mounted exactly as you stated. Everything looked fine. In researching the purchase of a new rearend, I learned of the difference in spring perches. It was then that I realized that my vehicle had the later style perches. Again, in looking at it, you wouldn't have thought there was anything wrong, but in direct comparison to a correct rear, it was obviously an issue. My pinion angle issue is solved now. Simply loosening the upper control arms revealed quite a bit of tension, and relieved a lot of it. I tried a couple sets of used springs and found ones that were slightly shorter. I set the weight onto the suspension, then tightened the upper arms. My pinion angle is now in spec and my only vibration is my 20 year old failing tires.
I have no idea what may or may not have been done to the Olds axle before it came to me. I do not know what the springs were.
I guess my original question was premature regarding the upper arms.
I fully understand the greater strength of the earlier Olds rear, but was asking for a relative comparison as upgrading from such a tall gear requires a carrier change and upgrade parts are expensive for that rarer rear.
It wasn't my intention to get into a comparison of knowledge with anybody.
I have a lot of experience, there's a load of people out there with a ton more than me.
Thanks again for everybody's input.
So far my plan has been to order some Rallye 1 wheels (15x7). I have a pair of 2456015 that I'm going to try on the front. If they go I'll get 2 more for the rear. If not I'll get a pair of 2356015 for the front and go with them staggered.


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## Mine'sa66 (Oct 30, 2019)

Mine'sa66 said:


> Hi,
> You were getting across just fine.
> I'm unclear where I stated that the spring perches should be on top of the tube. My spring perches were mounted exactly as you stated. Everything looked fine. In researching the purchase of a new rearend, I learned of the difference in spring perches. It was then that I realized that my vehicle had the later style perches. Again, in looking at it, you wouldn't have thought there was anything wrong, but in direct comparison to a correct rear, it was obviously an issue. My pinion angle issue is solved now. Simply loosening the upper control arms revealed quite a bit of tension, and relieved a lot of it. I tried a couple sets of used springs and found ones that were slightly shorter. I set the weight onto the suspension, then tightened the upper arms. My pinion angle is now in spec and my only vibration is my 20 year old failing tires.
> I have no idea what may or may not have been done to the Olds axle before it came to me. I do not know what the springs were.
> ...


So the 2456015 on 15x17 4.25 back space clear the front! Not a ton to spare, but it works. Looks pretty sharp with that size on Rallye 1's. Now the big question...Rallye 1's with or without trim rings?


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