# GTO Rear End Identification help????



## FireFox3434 (Jan 3, 2014)

Hi All:

Need help figuring out what I have for a rear end. CN4 is on the lower pumpkin and its a 10 bolt. this is on a 1968 GTO.

Thanks for any help.

Tom:smash:


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## 68GTO4004Spd (Jun 19, 2008)

Well if it looks like this, then it's a BOP rear end. To identify, look on the axle tube to the right of the pumpkin, should be about 6" to 12" away 2 letters about 1/2" tall and will be something like ZH. Here's a link to the codes.
http://www.wallaceracing.com/axledata2.htm


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## FireFox3434 (Jan 3, 2014)

*1968 gto rear end help identification*



68GTO4004Spd said:


> Well if it looks like this, then it's a BOP rear end. To identify, look on the axle tube to the right of the pumpkin, should be about 6" to 12" away 2 letters about 1/2" tall and will be something like ZH. Here's a link to the codes.
> http://www.wallaceracing.com/axledata2.htm


What is a BOP?


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## 68GTO_MAN (Dec 1, 2013)

FireFox3434 said:


> What is a BOP?


Buick olds pontiac


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## FireFox3434 (Jan 3, 2014)

Hi here are some pics of rear

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## PontiacJim (Dec 29, 2012)

Pontiac did not add the rear anti-roll bar until the 1970 model. You have a 1968, so either yours has been added on to the original rear, or the rear end has been swapped to include the sway bar set-up. 

68GTO4004sp provided the link you need to identify a Pontiac rear. Find the axle code on the axle tube as noted in the article. The numbers you provided may simply be part numbers or even foundry numbers. Get a copy of your car's build sheet from PHS services which will help identify the original rear end as well as all other codes, parts & options.


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## FireFox3434 (Jan 3, 2014)

I have the PHS documentation. 

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## FireFox3434 (Jan 3, 2014)

Found GAC 035 2 on one tube in front . If your looking from back of car its on the right in front tube.

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## PontiacJim (Dec 29, 2012)

FireFox3434 said:


> Found GAC 035 2 on one tube in front . If your looking from back of car its on the right in front tube.
> 
> Sent from AutoGuide.com Free App


Using the forementioned Wallace Rear End Codes provided earlier, it looks like G=Detroit Gear&Axle, AC=1970 Grand Prix, non-posi, 2.78 axle ration. Assume the 03=March and 5 may be 5th day, 2=night shift. Grand Prix's probably had the rear sway bar in 1970.

How does that match with your PHS documents as being the original?


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## FireFox3434 (Jan 3, 2014)

PontiacJim said:


> Using the forementioned Wallace Rear End Codes provided earlier, it looks like G=Detroit Gear&Axle, AC=1970 Grand Prix, non-posi, 2.78 axle ration. Assume the 03=March and 5 may be 5th day, 2=night shift. Grand Prix's probably had the rear sway bar in 1970.
> 
> How does that match with your PHS documents as being the original?


Thanks Jim now I get how to decode. It didn't match that's why I needed to know what it was.


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## PontiacJim (Dec 29, 2012)

FireFox3434 said:


> Thanks Jim now I get how to decode. It didn't match that's why I needed to know what it was.


Yep, can sometimes confusing, even for me. Glad to help. Good gas mileage/highway cruising gear as a good 400 has the torque to turn them over. If you do a lot of city driving/stop&go, then your gas mileage will be poor because it takes more gas pedal to get going after each stop, especially with an automatic and torque converter slip, which I assume you have with those gears (unless they have been changed).

Factory tire height was around 26", so if you put taller tires on your car, it can make them more like 2.56's which is worse for stop/go driving. Great for highway blasts as my brother had a '68 Bonneville with gears like which would downshift into second gear at 90 mph and keep on pulling.


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## FireFox3434 (Jan 3, 2014)

PontiacJim said:


> Yep, can sometimes confusing, even for me. Glad to help. Good gas mileage/highway cruising gear as a good 400 has the torque to turn them over. If you do a lot of city driving/stop&go, then your gas mileage will be poor because it takes more gas pedal to get going after each stop, especially with an automatic and torque converter slip, which I assume you have with those gears (unless they have been changed).
> 
> Factory tire height was around 26", so if you put taller tires on your car, it can make them more like 2.56's which is worse for stop/go driving. Great for highway blasts as my brother had a '68 Bonneville with gears like which would downshift into second gear at 90 mph and keep on pulling.


Jim:

The 68 has. 4 speed in her and can the tubes be from one car and that a different Pumkin center from another?? That has a NF on the pumpkin looked up numbers it shows from a 1970 Chevelle. If that works great just need what parts for rebuild lol. She is a project save her from a field rotting away.

Thanks for the help.

Tom


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## PontiacJim (Dec 29, 2012)

FireFox3434 said:


> Jim:
> 
> The 68 has. 4 speed in her and can the tubes be from one car and that a different Pumkin center from another?? That has a NF on the pumpkin looked up numbers it shows from a 1970 Chevelle. If that works great just need what parts for rebuild lol. She is a project save her from a field rotting away.
> 
> ...


Tom,

Hmmm, that would be way too much work in my opinion and not too practical. If the original gears are still in the rear end, I don't think you would be able to take off without a lot of clutch slip and I don't think you could pull the engine down to idle speed in gear without stalling it out. Does it still have the 4-speed/pedals/linkages or has someone attempted a trans swap with an automatic to go along with that rear end?

I am no expert, but the part number may be somewhat generic (called corporate) in that it is a GM piece. I found some interesting posts and info on the web that seem to indicate that there was a change-over between 1970-1971 from the 8.2" and 8.5" 10-bolt rear and strange things can be found. Some more identification is in order. 

Found a good ID and info at http://www.crankshaftcoalition.com and type in "Rearend identification" in their search engine. Has code identification and links to other sites.

Found this on a blog as well. Note the BOP (Buick/Olds/Pontiac) has bolts that hold the axle ends in place -take the drums off to see these 4 bolts located around the axle shaft:

Well,there are two basic kinds of 8.2" ten bolt rears.

There are the chevy style 8.2 rears w/c-clips.

And the BOP style 8.2 rears w/bolt in axles.

Almost nothing readily interchanges between those two rears.

And then there is the 8.5" corporate ten bolts,and again nothing will readily interchange between the 8.5 and the 8.2 rears.

The chevy 8.2 rear has a 1.438" dia. pinion gear,with 25 splines input for the yoke,it uses 10 ring gear bolts that are 3/8" x 24 RH threads.

The BP (buick/pontiac) 8.2 rear also has a 1.438" dia. pinion gear,but it uses a 27 spline input for the pinion/yoke,they also use the same 10 - 3/8" x 24 RH ring gear bolts that the 8.2 chevy diff does.

The 8.5 rears use a 1.626" dia. pinion,and they use 30 splines for the pinion/yoke,and they use 10 - 7/16" x 20 LH thread bolts for the ring gear.


What is your ultimate goal here? Are you looking to rebuild the 10-bolt, replace/upgrade, or simply identifying it?

Jim


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## FireFox3434 (Jan 3, 2014)

PontiacJim said:


> Tom,
> 
> Hmmm, that would be way too much work in my opinion and not too practical. If the original gears are still in the rear end, I don't think you would be able to take off without a lot of clutch slip and I don't think you could pull the engine down to idle speed in gear without stalling it out. Does it still have the 4-speed/pedals/linkages or has someone attempted a trans swap with an automatic to go along with that rear end?
> 
> ...



Hi Jim

Thanks for all the info. The ultimate goal is to figure out year and what it is so I can start getting parts to rebuild it. Look at my page I have 3 pics of my car. She is rough but got nothing but time. Bought a full trunk pan inner and outer rear wheel Housings and also ordered full quarters. She will be looking like the Bell of the Ball before I know it lol.


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## PontiacJim (Dec 29, 2012)

OK, I see. Looked at the pics. Rough project, but the beauty of it is you can get so many parts for the GTO/Lemans/Tempest. I had a '68 GTO. Still have the Pontiac plate from off the cowl. I like the '68 because last year of the vent window on the door. For my project '68 Lemans, I bought a one piece 1969 fiberglass nose (less the hood for my project} from VFN Fiberglass. I am going tilt nose. I will modify the '69 marker lights so as to be '68 correct. Also got their fiberglass rear bumper because I am going paint versus chrome. Long term project just like everybody else, but when done, it will be my creation. Keep us posted and add pics of your progress.


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