# 64 GTO rear end change



## boomer128 (Nov 7, 2010)

I recently bought a 64 GTO tri-power with a close ratio 4sp. I just completed the rear wheel/drive shaft turn test and it showed a little over 4 turns so I assume I have a 4.11 rear end gear. I would like a higher gear so I could cruise at 70 peacefully and at lower RPM's.What gear would you suggest and what do I need to make a change?
Thanks


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## Pinion head (Jan 3, 2015)

boomer, sounds like an interesting find, congrats!

On the rearend, first have to ID what rear is in the car. the original gray iron 8.2 Pontiac 10 bolts often were swapped out. There is a strong consensus of opinion that a close ratio Muncie 4spd was not avail from the factory in a '64 GTO so there is a good possibility that the transmission, if a M21 was swapped, and either the ring and pinion changed, or the whole rearend could have been changed. There are numerous styles of '64-72 GM A-body rearends that can fit, or be easily swapped into a '64 with a few mods. Below is a link to a '64 rearend question last week. 

http://www.gtoforum.com/f50/64-gto-rearend-numbers-92617/

The easiast way, I can ID is by the casting number on the center housing, just on Pontiac 8.2 10 bolts have a list of 14 different casting numbers. After IDing rearend style, can provide better info.


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## boomer128 (Nov 7, 2010)

A lot of work was done to this car, rebuild tri-power, rebuild motor, disc brakes and 4 sp close ratio. Undoubtedly the rear end has been changed. The only ID I have found on the 10 bolt pumpkin is H27 in real small type. Mean anything to you?
Thanks for your response.


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## Pinion head (Jan 3, 2015)

the casting number will be on one of the angled support areas from the center housing to the axle tube. On a '64, have to clean it real well, I use a rotary wire brush. The '64 8.2 Pontiac 10 bolt rear, the casting number is not raised, but debossed... almost like the casting numer is stamped into the metal. Pontiac central foundry quit doing that and later casting numbers are raised.

if you can get some pics from the rear cover side, it would help, also of how the rear coli springs are located at the perch. Last, does the rearend use ubolts with nuts to attach the rear ujoint to the pinion flange, or stamped steel straps and 7/16" head bolts?

will ck in late tonight.


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## geeteeohguy (Feb 2, 2008)

H27 would be an August 2, 1967 date code....usually. The carrier you have in your rear end will be able to accomodate a 3.36 gear, which would get you where you want to be. If you want to go numerically lower, you'll need to change out the carrier. You simply need a ring and pinion, and a competent technician to do the swap. But, as stated, determine what you have before you go hunting for parts. Pinion head is the go-to guy for this type of information, so you are lucky he's involved here!


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## boomer128 (Nov 7, 2010)

geeteeohguy said:


> H27 would be an August 2, 1967 date code....usually. The carrier you have in your rear end will be able to accomodate a 3.36 gear, which would get you where you want to be. If you want to go numerically lower, you'll need to change out the carrier. You simply need a ring and pinion, and a competent technician to do the swap. But, as stated, determine what you have before you go hunting for parts. Pinion head is the go-to guy for this type of information, so you are lucky he's involved here!


Thank you for the response. I'll do some more checking under the car to see what other info I can find.
Thanks again.


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## boomer128 (Nov 7, 2010)

boomer128 said:


> Thank you for the response. I'll do some more checking under the car to see what other info I can find.
> Thanks again.


Did some more checking. It is a 10 bolt, with a single flange/gusset on each side of the housing headed toward the u joint. On the bottom of the housing is a raised stamped number which is either 9779322 or 9779822. Will this ID the rear end?

What has to be done to change the gearing/
Thanks


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## boomer128 (Nov 7, 2010)

Pinion head said:


> the casting number will be on one of the angled support areas from the center housing to the axle tube. On a '64, have to clean it real well, I use a rotary wire brush. The '64 8.2 Pontiac 10 bolt rear, the casting number is not raised, but debossed... almost like the casting numer is stamped into the metal. Pontiac central foundry quit doing that and later casting numbers are raised.
> 
> if you can get some pics from the rear cover side, it would help, also of how the rear coli springs are located at the perch. Last, does the rearend use ubolts with nuts to attach the rear ujoint to the pinion flange, or stamped steel straps and 7/16" head bolts?
> 
> will ck in late tonight.


I did some more checking. It is a 10 bolt with one flange/gusset on each side of the housing headed toward the u joint. Stamped/raised on the bottom of the housing is the number 9779322 or 9779822. Can you tell what rear end it is from this info? 

What has to be done to change the gearing? Thinking 3.36.
Thanks for your time.


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## geeteeohguy (Feb 2, 2008)

The ring and pinion gears need to be changed out. I recommend you have a differential shop do this, or a pro. Not a place for the uninitiated. Not a big job, but it takes special tools and expertise. Get it wrong and the rear end turns into scrap metal very quickly.


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## Pinion head (Jan 3, 2015)

The casting number 9779822 is going to be from a '65 Pontiac 8.2 10 bolt, possibly an early '66 8.2 Pontiac rear. The first '66's were the same width as the the narrow '64-65 Pontiac 8.2 rears. A little later in '66 production, the slightly wider Pontiac 8.2 rears were introduced. 

If the rear was about to go on my build bench, I'd ck out what posi is in it. Also if axle bearings are originals, or have been replaced. Sealed axle bearings are ticking time bombs and eventually seize and chew up the axle, so I nearly always replace 'em. On the posi unit, it maybe an original two pinion posi or a later 4 pinion posi, or an Eaton or Auburn. With a low ratio Pontiac 8.2 carrier, 3.36 is as a high a ratio as you can go. With 27" tall tires, 65mph is bareable, but with 25 1/2 to 26" tall tire/ wheel combo, the engine is going to buzzing right along. What does the '64 have for an engine? Modest cammed 389 stocker, or something quite a bit stronger? Trying to understand the close ratio Muncie with a set of 3.36's.


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## boomer128 (Nov 7, 2010)

Pinion head said:


> The casting number 9779822 is going to be from a '65 Pontiac 8.2 10 bolt, possibly an early '66 8.2 Pontiac rear. The first '66's were the same width as the the narrow '64-65 Pontiac 8.2 rears. A little later in '66 production, the slightly wider Pontiac 8.2 rears were introduced.
> 
> If the rear was about to go on my build bench, I'd ck out what posi is in it. Also if axle bearings are originals, or have been replaced. Sealed axle bearings are ticking time bombs and eventually seize and chew up the axle, so I nearly always replace 'em. On the posi unit, it maybe an original two pinion posi or a later 4 pinion posi, or an Eaton or Auburn. With a low ratio Pontiac 8.2 carrier, 3.36 is as a high a ratio as you can go. With 27" tall tires, 65mph is bareable, but with 25 1/2 to 26" tall tire/ wheel combo, the engine is going to buzzing right along. What does the '64 have for an engine? Modest cammed 389 stocker, or something quite a bit stronger? Trying to understand the close ratio Muncie with a set of 3.36's.


The engine is a bit of a mystery also. The engine code is 493751YC. The tri-power and engine were rebuilt in 2013. At the same time the 4sp close ratio was installed.

I am still trying to figure out if I have a 389 or 400 cid.


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## Pinion head (Jan 3, 2015)

With a 3.36 gear, the lowest numerical gear avail on the low ratio carrier, you will be looking at right under 3000rpm cruising rpm at 70 mph with 26.75" tall tire wheel combo. 

The problem Im betting you are going to experience with the higher ratio rear ratio and the close ratio Muncie is the torque multiplication of the gearbox is not going to be that good, 2.20 x 3.36= 7.39 overall first ratio, optimum on the street is typically a number between 8.5 and 9.0.

With a 3.90 and an M21, overall first gear ratio is 8.54
a 3.73 and a 2.20 1st gear Muncie, overall first gear ratio is 8.21
a 3.36 and a 2.52 1st gear M20, overall first gear ratio is 8.46
a 3.23 and a 2.52 1st gear M20, overal first gear ratio is 8.14


When Pontiac did start offering 2.20 first gear boxes in conjunction with 455's and 3.31 12 bolt gears, there were some warranty problems with clutch discs from owners who were slipping the clutch. the overall first gear ratio with that combo was only 7.28. 

Depending on your intended use, would really spend some time thinking out the transmission and rear gear ratio.


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