[JBates]

Hi everyone I recently posted a thread requesting recommendations on a hydraulic flat tappet cam for my engine build. Based on the responses I now have a pretty good idea of what would be suitable. In the process I found out I have a 2.56 Rear End. Based on my build and intended use I’ve been advised I’ll need to go with 3.42 or 3.55 gears.

After doing some more digging it looks like I have a oldsmobile 12 bolt cove / 10 bolt carrier on my car. One of the prior owners swapped out the original engine with a ’73 400 and 4x low compression heads. In the process they must have replaced the rear end and went with the lazy 2.56 rear end.

Checking to see if I can get some feedback on the most feasible way to to get my desired rear end. Should I look into getting a complete different rear end or is it possible to get rebuild kit for the 12 bolt oldsmobile rear end with 3.42 or 3.55 gears.

I’m hoping to avoid any wheel spacing offsets so ideally it would be nice if I can find a 1966 Pontiac GTO rear end. Are there any good alternatives? Any feedback is greatly appreciated.

1966 Pontiac GTO
-‘73 400 ci block going to be bored out 0.30 over from 4.12 to 4.15
-Tri Power Carbs 
-‘68 #16 heads 2.11/1.77 valve heads (72-75cc)
-14cc dish pistons
-CAM Crower 60243 or Voodoo 703
-Ram Air Exhaust Manifolds
-2.5" Exhaust 
-4 Speed Muncie M20

Thank you

[lust4speed]

Have two friends with the Olds 12 bolt. Both have pulled their hair out trying to find replacement parts for them. First friend gave up and had me swap in a normal BOP 10-bolt rear and not being a racer the Pontiac 10-bolt will last him forever. He left the Olds rear with me and before I could dispose of it the other friend needed an axle and couldn't find a source. I gave him the rear and he then found out that Olds went to a different taper on the inner splines and the donated axles wouldn't slide in his unit. Spline count was the same and his old good axle would slide into the fubar'd donated rear but not the other way around.

Personally, I'd go with any other rearend and run away from the Olds because sometime, somewhere you won't be able to find parts for the orphan.

[67drake]

I agree with lust4speed. I would ditch the Olds.
Many options, but just a question of how you want to do it. The early 70’s GM A body 10 bolt I think is 1/2” wider than the original that was in your ‘66, but they are strong and parts available. Nothing wrong with a 8.2 original A body rear, but some poo poo them on being weaker.
Where are you located?

[400 Lemans]

https://www.ronsmachiningservice.net...posi-gear-pkg/ If you decide to keep the olds 12bolt.

[JBates]

Originally Posted by 67drake

I agree with lust4speed. I would ditch the Olds.
Many options, but just a question of how you want to do it. The early 70’s GM A body 10 bolt I think is 1/2” wider than the original that was in your ‘66, but they are strong and parts available. Nothing wrong with a 8.2 original A body rear, but some poo poo them on being weaker.
Where are you located?

Hi everyone, thanks for the input.

67drake, I'm in San Diego. Last night I found a Pontiac Auto Dismantler who is selling a 1968 GTO, LeMans, Grand Prix 3.55 Posi diff assy (ZH 291 N #9798896). He noted it looks good but has a chipped tooth on the ring gear and is asking $950.

I'm going to start will calling all teh local junk yards, craigslist ect. I'm sure they are out there, it's just going to be a matter of luck and timing.

Thanks again

[JBates]

Originally Posted by JBates

Hi everyone, thanks for the input.

67drake, I'm in San Diego. Last night I found a Pontiac Auto Dismantler who is selling a 1968 GTO, LeMans, Grand Prix 3.55 Posi diff assy (ZH 291 N #9798896). He noted it looks good but has a chipped tooth on the ring gear and is asking $950.

I'm going to start will calling all teh local junk yards, craigslist ect. I'm sure they are out there, it's just going to be a matter of luck and timing.

Thanks again

I forgot to note, the same guy selling the 1968 GTO, LeMans, Grand Prix 3.55 Posi diff assy also has a 1966 GTO, LeMans 2.93 Open Differential Assy $525. He's listing it as in great condition, complete & ready to go.

The posi is definately more appealing due to the 1966 rera end wouold need to have the gears changed.

[Tom Vaught]

Originally Posted by 67drake

I agree with lust4speed. I would ditch the Olds.
Many options, but just a question of how you want to do it. The early 70’s GM A body 10 bolt I think is 1/2” wider than the original that was in your ‘66, but they are strong and parts available. Nothing wrong with a 8.2 original A body rear, but some poo poo them on being weaker.
Where are you located?

Going by memory, I believe the later rear axle is 1" total wider and 1/2 inch wider in each direction. Your post assumes 1/4" wider per side.

Tom V.

[Singleton]

66-72 pontiac A-body rear ends are same width. 64-65 are 1" narrower. Some say the early build 66 rears are also narrow, (like the 64-65), but so far I've never seen anything other than anecdotal evidence to back that, (no date codes or cast # with actual apples-to-apples measurements).

[JBates]

Thank you everyone for the responses and feedback on my post.

If $$$ wasnt a factor and since I'm not looking for originallity the obvious route would be to go with a 12 bolt Chevy or aftermarket 9 inch.

Since my motor will be a fairly stock build (360 - 375 HP) I'm leaning towrads keeping the 8.5 10 bolts olds or possibly continuing my search for an original 8.2 BOP 10 bolt.

I plan on lowring my 66 GTO a couple inches and looking to go with 18 x 9" wheels on the rear. That being said I would prefer the stock rear end width in order to help avoid any rubbing on the fender walls.

When I removed the drive shaft I was only able to slide the drive shaft forward only about 1/2" or so which made it difficult to get the rear u-joint to come out. I ended up having to remove the u-joint caps and having to lightly pry it out. Does this sound like the prior owner didnt shorten the drive shaft or did they not shorten it enough?

Greg Reid, my rear u-joint was also retained to the olds pinion yoke by nothing but the u-bolts as well.

I'm located in San Diego CA and having a heck of a time finding an original 8.2 BOP 10 bolt. Franks Pontiac Parts & Auto Dismantling which is about 3 hours away has a 1966 GTO, Lemans 2.93 Open Diff Assy for $525. Its listed as in great condition, complete & ready to go. If I go this route I still need to replace the gears with what I'm looking for and will need to search for a suitable kit.
This is the same guy selling the 1968 GTO, LeMans, Grand Prix 3.55 Posi diff assy (ZH 291 N #9798896) with the chipped tooth on the ring gear and is asking $950. Based on the feedback I will stear clear of this one.

If I stay with my current 8.5 10 bolts Olds rear I might got with 400 Lemans suggestion
https://www.ronsmachiningservice.net...posi-gear-pkg/
Looks like this kit "GM 8.5" Olds "O" Axle 3.42 Ring & Pinion 28 Spline Posi Gear Pkg" runs at $940.

OG68 recommended an Differential and Axle shop which is located in a town right next to mine. I was planning on rebuilding the axel myself; however, I just might visit that shop to see about getting some additional feedback and an estimate.

In the meantime; thanks again everyone for the suggestions. Any additional feedback is greatly appreciated.

BTW if anyone is SoCal is intersted in selling a 8.2 BOP 10 bolt for a 1966 GTO, please hit me up.

Thank you

[The Champ]

Originally Posted by JBates

I plan on lowring my 66 GTO a couple inches and looking to go with 18 x 9" wheels on the rear. That being said I would prefer the stock rear end width in order to help avoid any rubbing on the fender walls.

With your plans of going to 18x9" wheels, the rear end width doesn't matter. You have to buy new rims, so just buy rims with the proper backspacing.

In my '64, it had the 1" wider 10 bolt rear end in it when I bought it. With factory 15x7" GM rims with 4.25" BS, I was able to fit 255/60/15's on the rear with adequate clearance. I now run 17's and still running the same width rear end (now a 12 bolt).

Going to an 18x9" rim would require 1" more BS to center that same width tire in the exact same place in the wheel well. If you are going with a wider tire, you just need to add .5" of BS for every 1" of tire width.

Summit currently has 41 different 18x9" wheels with between 5.25 and 6" of BS, so wheel choices are more than abundant. If the style you want isn't among those 41 choices, many wheels are available with custom BS.

[drewm]

Originally Posted by JBates

Thank you everyone for the responses and feedback on my post.

If $$$ wasnt a factor and since I'm not looking for originallity the obvious route would be to go with a 12 bolt Chevy or aftermarket 9 inch.

Since my motor will be a fairly stock build (360 - 375 HP) I'm leaning towrads keeping the 8.5 10 bolts olds or possibly continuing my search for an original 8.2 BOP 10 bolt.

I plan on lowring my 66 GTO a couple inches and looking to go with 18 x 9" wheels on the rear. That being said I would prefer the stock rear end width in order to help avoid any rubbing on the fender walls.

When I removed the drive shaft I was only able to slide the drive shaft forward only about 1/2" or so which made it difficult to get the rear u-joint to come out. I ended up having to remove the u-joint caps and having to lightly pry it out. Does this sound like the prior owner didnt shorten the drive shaft or did they not shorten it enough?

Greg Reid, my rear u-joint was also retained to the olds pinion yoke by nothing but the u-bolts as well.

I'm located in San Diego CA and having a heck of a time finding an original 8.2 BOP 10 bolt. Franks Pontiac Parts & Auto Dismantling which is about 3 hours away has a 1966 GTO, Lemans 2.93 Open Diff Assy for $525. Its listed as in great condition, complete & ready to go. If I go this route I still need to replace the gears with what I'm looking for and will need to search for a suitable kit.
This is the same guy selling the 1968 GTO, LeMans, Grand Prix 3.55 Posi diff assy (ZH 291 N #9798896) with the chipped tooth on the ring gear and is asking $950. Based on the feedback I will stear clear of this one.

If I stay with my current 8.5 10 bolts Olds rear I might got with 400 Lemans suggestion
https://www.ronsmachiningservice.net...posi-gear-pkg/
Looks like this kit "GM 8.5" Olds "O" Axle 3.42 Ring & Pinion 28 Spline Posi Gear Pkg" runs at $940.

OG68 recommended an Differential and Axle shop which is located in a town right next to mine. I was planning on rebuilding the axel myself; however, I just might visit that shop to see about getting some additional feedback and an estimate.

In the meantime; thanks again everyone for the suggestions. Any additional feedback is greatly appreciated.

BTW if anyone is SoCal is intersted in selling a 8.2 BOP 10 bolt for a 1966 GTO, please hit me up.

Thank you

I am no expert on rears. There are several experts on this boards, and they frequently post and there is a ton of info available if you search on this site. However, it sounds (and looks) like you have an Olds Type "O" rear, which were made from 68-70, and are marginally stronger than a standard 8.2. However, they are not as strong as a 8.5 10 bolt from an olds or buick from 71-72. If you are going to drop a grand or more and rebuild a rear, I think a better starting point is the olds/buick 8.5 from 71-72. There are good 8.2's out there that would fit your car, but unless you rebuild, you are putting an unknown under your car.

[geeteeohguy]

Originally Posted by drewm

I am no expert on rears. There are several experts on this boards, and they frequently post and there is a ton of info available if you search on this site. However, it sounds (and looks) like you have an Olds Type "O" rear, which were made from 68-70, and are marginally stronger than a standard 8.2. However, they are not as strong as a 8.5 10 bolt from an olds or buick from 71-72. If you are going to drop a grand or more and rebuild a rear, I think a better starting point is the olds/buick 8.5 from 71-72. There are good 8.2's out there that would fit your car, but unless you rebuild, you are putting an unknown under your car.

This is solid advice you can take to the bank. No reason to run an 8.2 unless it's cheap and in good shape or just a cruiser. I have them in both my cars but have broken them in my youth more than once. A member here, Ol'Pinion Head or Pinion Head is THE authority on rear end swaps and what works best on these cars. He's been parting/fixing/driving these cars for decades and has all the information you could possibly need.

[JBates]

Here are some images of the 3.55 posi with the chipped tooth. Is replacing the ring gear a must or do you think I could get bye with using it as is?

Thank you

[mgarblik]

Originally Posted by JBates

Here are some images of the 3.55 posi with the chipped tooth. Is replacing the ring gear a must or do you think I could get bye with using it as is?

Thank you

It is very unlikely that that ring gear tooth chipped off from acceleration or lack of lube. A single tooth chip like that almost always comes from another piece of broken crap getting stuck between the ring and drive pinion gear. Then something has to give and it's usually a ring gear tooth. I would expect drive pinion damage, or side gear damage as well. Plan a complete overhaul if buying that unit. Personally, I would pass on it. Lots of expensive parts in there and lots of labor as well. 4-5 hours labor to disassemble, clean and set up new gears.

[drewm]

My 66 GTO is a long long way from being done, but I found a 8.5 10 bolt out of a 71 olds cutlass that I am going to rebuild and use in my 66. If you are going to rebuild and are not worried about originality, a rear from an A body olds or buick from 71 or 72 will work, but you may have to shorten the driveshaft. Other than that, they are a stronger rear than the 8.2 and because they became the GM corporate rear for many years, plenty of aftermarket gearsets and parts are available. The rear in my 66 had a busted up pinion gear and a welded upper control arm ear, so instead of rebuilding a rear with a questionable history, I found this 8.5 rear for $150 bucks. Maybe not as strong as an aftermarket 9 inch or 12 bolt, but plenty strong for what I plan. I will still need to spend money on the rebuild, but I should be well ahead of an aftermarket rear.

[Singleton]

Originally Posted by drewm

…I found a 8.5 10 bolt out of a 71 olds cutlass that I am going to rebuild and use in my 66.

Agree
The 8.5 from a 71-72 olds/buick a-body is the better way to go.

[OG68]

Call Jesse or Joe up at the Differential and Axle shop in Escondido. He’s rebuilt the diffs in my el Camino, GTO and Silverado. Good people, fair prices.

http://www.thedifferentialandaxleshop.com/

[Greg Reid]

I'm in the process of replacing the Olds rear in my 68 with a Pontiac 10 bolt. It's a 2.78 peg leg. It's free if someone wants to pick it up. Nothing wrong with it, just going to a 3.55 nodular limited slip and l don't have a use for it.

[Greg Reid]

By the way JBates, how was the u-joint retained on the Olds pinion yoke? Mine was retained by nothing but the u-bolts and no way to precisely locate it without clips. There is a mismatch between the Olds pinion yoke and the Pontiac drive shaft yoke as far as retaining clips used.

[Mike Davis]

If you are not concerned with an original look, I would look for a 12 bolt (esp with a 4 speed). Stronger, easier to find gear sets/posi units etc. 8.2/8.5 will work for a while, but if you ever get good traction it may not live ling behind a 4 speed.