# Identifying the Rear Diff



## Jim (Oct 30, 2008)

Hey Guys, 

I finally got the correct gears for the rear end locally once I got the old ones out. I used Rukee's method to initially identify what I had in the car (2.56) and it was absolutely accurate (Nice Trick Rukee, thanks)

What I found is I have a 8.5" rear end with no C-Clips. The axles bolt at the hubs. From what I could tell it appears that this may be an early 70's vintage rear end. I couldn't find any numbers on it either. There are some numbers on the ring and pinion but I don't know if there is way to cross reference.

Anyone know how I might identify this?

Thanks

Jim


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## Tri-Power (Jan 10, 2008)

sounds like you quite possibly could have an original, or close to original rear end in the car. Here is the thread I gfot going when I was trying to ID my '66 axle housing....

1966 differential ID - Pontiac GTO Forum

hopefully that helps a little.


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## Jim (Oct 30, 2008)

Well, It makes me feel a little better that it may be the correct rear end. Did you have c-clips on your axles or were the held in with plates at the drums.

Also, I had to change the carrier because the one I had in there would not work with the lower gears. I went from 2.56 to 3.42. I don't understand why anyone would have the 2.56's. Regardless, since I had to change the carrier I went with a posi, better burn outs.

Does anyone have any need for my old gears and carrier?

Let me know.


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## geeteeohguy (Feb 2, 2008)

The correct rear end for your car will not have any c-clips for the axles. Pontiac axles are held in at the flange. Again, sounds like you have the "correct" rear end for your car. 2:56 gears came in 6 cylinder Tempests and also in V8 Tempest/leMans/GTO's as an "economy"gear, and also was used in air conditioned cars: lower revs=less engine heat. Great for a Highway cruiser, but not the best for a musclecar. My favorite ratio for both stick and auto GTO's is a 3.23. I have 3.36 gears in both of mine, and they work well all-around.


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## Tri-Power (Jan 10, 2008)

Like Jeff said, mine also has the bolt-in flanges. None of that c-clip nonsense for us!


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## 67gteeoh (Dec 17, 2008)

Jim said:


> Hey Guys,
> 
> I finally got the correct gears for the rear end locally once I got the old ones out. I used Rukee's method to initially identify what I had in the car (2.56) and it was absolutely accurate (Nice Trick Rukee, thanks)
> 
> ...



I too am changing the gears in my 67 and ran into a similiar problem. I ordered what I thought should be the correct gear set from Randy's which was a 8.2 BOP, 3.36 ratio to replace the 3.90's currently in the car. 
When my mechanic took the rear end apart, he measured the old ring gear at 8.5 with a 30 spline pinion which I believe puts it somewhere in the early 70's. I have not yet run the numbers......that's next. 


So now I'm confused, what I thought was the original rear end in the car turns out NOT to be. I am now suspecting that someone might have changed it with a newer year. 

So now it's back to Randy's to re-order the 8.5, hassle with the returns. I'll have to go with a 3.42 since they don't make 3.36's in a 8.5" ring gear.


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## Jim (Oct 30, 2008)

*3.42's*

That's what I wound up putting in mine and I love em. I don't know that I want to be running 80 mph with them but 65 to 70 is fine and low end is alot better !!!!!!!


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## 67gteeoh (Dec 17, 2008)

Jim said:


> That's what I wound up putting in mine and I love em. I don't know that I want to be running 80 mph with them but 65 to 70 is fine and low end is alot better !!!!!!!


It's a HUGE difference from the 3.90's and I love it!! The car is actually a pleasure to drive now. I do not have a tach so I'm unsure of the RPM at 65-70, but I must say it's alot better.


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