# Rear axle identification???



## ylwgto (Oct 27, 2009)

Hi,
So I am getting ready to install a rear disc kit on a rear differential that I thought was appropriate for my 1966 GTO. After a little more investigation, I am not sure what the year and gear ratio is. It is a 10 bolt posi rear, 56" flange to flange, and has parrallel ribs in the center section.
Codes are described below with their location. Any help would be greatly appreciated, as I want to make sure it is the rear I thought is was prior to putting time and money into it! Thanks.

Axle tube stamped code: 
GFC 259 2W (on front facing side of passenger axle tube)

center section cast codes:
GM60 (lower, passenger corner of pinion-side)
69227NF (lower, driver corner of pinion-side)
G or C ON2 (lower driver side corner of inspection cover side )
H51 (lower passenger side corner of inspection cover side)

There is also "12345 67890" cast into the inspection drivers side of the center section


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## 05GTO (Oct 6, 2004)

ylwgto said:


> Hi,
> So I am getting ready to install a rear disc kit on a rear differential that I thought was appropriate for my 1966 GTO. After a little more investigation, I am not sure what the year and gear ratio is. It is a 10 bolt posi rear, 56" flange to flange, and has parrallel ribs in the center section.
> Codes are described below with their location. Any help would be greatly appreciated, as I want to make sure it is the rear I thought is was prior to putting time and money into it! Thanks.
> 
> ...


I can't find any info on your codes, Does your diff cover look like this?










Here is a pictures of the BOP ribs;


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## ylwgto (Oct 27, 2009)

*here are pics*

of the rear end. it does not have the scalloped edges like the 8.2 Pontiac rears I have seen. Thanks.


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## 05GTO (Oct 6, 2004)

It looks like a GM 8.5 used after 1970 in Chevy, Buick, Olds and Pontiac,

Here is a list of cars that used the 8.5"

1971-'76 Chevrolet full-size cars
1994-'96 Impala
1971-'81 Buick full-size cars
1973-'77 Buick Gran Sport
1972-'77 Pontiac GTO and Le Mans
1970-'81 Chevy Camaro and Z28
1970-'81 Pontiac Firebird
1971-'77 Olds Cutlass and F-85 and Pontiac Grand Prix
1971-'79 Chevy II, Nova, Buick Skylark and Olds Omega
1973-'77 Chevelle, Malibu and El Camino
1971-'77 Pontiac Ventura and Phoenix
1971-'81 Pontiac Bonneville
1973-'77 Monte Carlo
1973-'77 Buick ******, Regal and Century
1980-'94 Chevy ½-ton and ¾-ton pickups
1977-'94 GMC Jimmy and Chevy Blazer


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## 05GTO (Oct 6, 2004)

ylwgto said:


> Axle tube stamped code:
> GFC 259 2W (on front facing side of passenger axle tube)


Found this information;



> The rear axle ratio, differential type, manufacturer, and build date information is stamped on the right axle tube (as viewed from the front of the car) on the forward side. The first 3 digits indicate the axle code, the next one indicates manufacturer (B=Buick, O=Olds, C=Chevy, P=Pontiac), the next is Julian date it was built and the last it the shift it was made on. Limited slip rearshad a tag on the differential cover that stated it need posi fluid... that's usually missing, if the cover has ever been off. However, a "B" in the axle code (i.e., 2BX) usually indicates a posi.
> 
> The 7.5" rears have a differential cover that is smooth and kind of a squared off oval shape. The 8.5" rears have a rounder shaped cover with a raisedhorizontal area in the middle.


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## ylwgto (Oct 27, 2009)

thanks. i thought it couldn't be a '70 or later rear b/c it is only 56" flange to flange where as the later rears are 58" I think. I am pretty sure it is an A body rear due to the coil perches and control arm mounts.
anyone know a definitive test to determine if it is a chevy, pontiac or olds rear? the code doesn't sem to be helping me much...
if i open it to look at the posi unit will a clutch vs cone posi help date it? any other diagnostic i can do while it is open?
i am pretty sure it is bolt in axles rather than c clip, but how can i be sure?


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## Too Many Projects (Nov 15, 2008)

OK, the research I did on Team Chevelle indicates this SHOULD be an early 1972 8.2 axle. The 69277 matches a Chevelle 10 bolt but is missing the 39 prefix ? 
The H51 is a casting date code of August 5th 1971 for a 72 application. 
The GFC 259 2W breaks down like this; GF=3.08 ratio. C = Buffalo NY assembly plant, 259 is the 259th day of 1971 = Thursday, Sept 16th build date. 2 is second shift and I haven't found any reference to the W. Width info is below. 

1964-67 Chevelle rear ends were manufactured by the Chevrolet Gear and Axle Division. The housing measures 53.25 inches from backing plate to backing plate and with the brake drums in place, the total width is 60.00 inches. Axle tube flange-to-flange is 54.25 inches.


* 1968-72 Chevelle rear end housings measure 58.5 inches from flange to flange, and with the brake drums in place, the total width is 62.5 inches. (Note: Other sources put the total width at 61.125" to 61.25".) Axle tube flange-to-flange is 55.25 inches.

* A second measurement to check is centerline distance between spring perch seats. 1964-1967 are 35 1/16 inches between seats where 1968-1972 are 34 31/32 inches.

If you pull the cover and it has spring and clutch posi, it has been put in after manufacture. If it has cone posi, I'm lost ....


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