#21  
Old 12-05-2021, 04:19 PM
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AG AG is offline
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Originally Posted by b-man View Post
Had my ‘71-‘72 Buick/Olds corporate 8.5 10-bolt built by Monzaz and shipped to me in CA. It was built with a 30-spline Eaton posi and Moser axles along with 3.42 gears, something along those lines would be more than up to the task for a 400 Tri-Power 4-speed street GTO.

The nice things about that particular rear are the tapered axle bearings that are lubed with gear oil and the lack of C-clip axle retention like most other 8.5 corporate rears as well as 12-bolt rears. No need for the C-clip eliminator kits needed on 12-bolt rears that are known leakers.

The axles bolt in like on the much weaker 8.2 rears that have the failure-prone sealed axle bearings.

The 8.5 rears all have nodular iron housings which are far superior in strength to the grey iron 8.2 rear. There are nodular 8.2 rears but they’re quite rare and inferior to the 8.5 which uses the same robust pinion bearings as a 12-bolt, the lack of pinion support is why it’s not really worth investing in building a performance 8.2 rear.

The corporate 8.5 rivals the strength of a 12-bolt, they can take a beating.
I run a 1971 Nova 8.5" rear in my racecar with tubes welded to the center section, Strange spool, Moser 33 spline axles, Strange billet bearing caps and an LPW cover and it has held up flawlessly with 700 Hp. I've checked the guts several times and everything is good. A properly built 1971-1972 A-body 8.5" rear will more than adequate for any mild to wild street car, the ales are very beefy.

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1967 Firechicken, 499", Edl heads, 262/266@0.050" duration and 0.627"/0.643 lift SR cam, 3.90 gear, 28" tire, 3550#. 10.01@134.3 mph with a 1.45 60'. Still WAY under the rollbar rule.
  #22  
Old 12-06-2021, 03:48 PM
JBates JBates is offline
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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


Last edited by JBates; 12-06-2021 at 03:50 PM. Reason: misspelling
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  #23  
Old 12-06-2021, 04:32 PM
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geeteeohguy geeteeohguy is offline
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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
If you want to drive the car for any length of time faster than 55 or 60 mph, stay away from the 3.55-3.42 gears. 3.23 are about the perfect gear for a 4 speed A body, and 3.36 are decent too. I had the 3.55's removed from my '65 GTO (389/tripower/M20) and had 3.36 gears installed and that moved my cruise speed from 60 to 70 mph. Not much on paper, but the car got a whole lot more drivable. 3.55's suck at any speed over 60 mph for more than 5 miles.

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  #24  
Old 12-06-2021, 04:35 PM
JBates JBates is offline
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Originally Posted by geeteeohguy View Post
If you want to drive the car for any length of time faster than 55 or 60 mph, stay away from the 3.55-3.42 gears. 3.23 are about the perfect gear for a 4 speed A body, and 3.36 are decent too. I had the 3.55's removed from my '65 GTO (389/tripower/M20) and had 3.36 gears installed and that moved my cruise speed from 60 to 70 mph. Not much on paper, but the car got a whole lot more drivable. 3.55's suck at any speed over 60 mph for more than 5 miles.
Hi Geeteeohguy, I plan on doing allot of highway / freeway driving. Thanks for the feedback

  #25  
Old 12-07-2021, 08:43 AM
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The Champ The Champ is online now
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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.

  #26  
Old 12-07-2021, 02:48 PM
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geeteeohguy geeteeohguy is offline
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Hi Geeteeohguy, I plan on doing allot of highway / freeway driving. Thanks for the feedback
In your case, then, you can't go wrong with a 3.23 rear ratio with your 4 speed and car weight. All around perfect gear for a street driven stick car.

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  #27  
Old 12-07-2021, 06:08 PM
drewm drewm is offline
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Originally Posted by JBates View Post
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.

  #28  
Old 12-07-2021, 07:17 PM
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geeteeohguy geeteeohguy is offline
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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.

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