1966 GTO Rear End Help. Need to replace the 2.56 gears
 

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

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.

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?

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

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.

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

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.

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).

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.

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.

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

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/

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.

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.

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.

I went through a similar decision making process with my '66 a few years ago. The car came to me with a worn out Olds "Type O" rear end, but mine had the more desirable 31-spline 3.91 posi setup. It whined terribly under deceleration. I set up an account on ClassicOldsmobile.com and got connected with someone who rebuilds the Type O rear ends for a living (username: Monzaz) to inquire about its value. He said as a core in its current state needing rebuild, it was worth $500. To fully rebuild it would add up and ultimately be less durable than other non-original options (Quick Performance 9" or Strange S60). Anyway, my crusty worn out Type O rear end was quickly snapped up for $500 by a ClassicOlds forum member, so I was happy to see it be saved from being discarded as scrap. Had a bomb-proof Quick Performance 9" built for ~$2500. It's awesome.

I share the opinion of not reusing your Type O rear end in your '66. Find and Oldsmobile guy who wants it and have something else built to your specifications.

A 2.75 first gear set in the turbo 400 would really work well. You'll love cruising the highway at 110-120 with the engine loafing at 2500 rpm..

Monzaz is on PY Forums too. Very knowledgeable and helpful. Here’s his website:
http://www.jdrace.com/

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.