Non stock rear end

[jleslein]

1975 Grand Am; purchased in February, now tackling rear drum brakes. The parts in kit purchased for this yr/make/model doesn’t match parts in rear end! In addition I found that differential gasket I bought earlier was 10 bolt like I have, but different pattern. My brother says a previous owner likely swapped in a different rear end. Now the $64 question; how to proceed? Maybe convert to disc to bypass whole issue? Not sure how to I.D. exactly what rear end I have? Thanks for any info!

[ctesla]

hey jl,
welcome to Automotive Forums..
your brother is likely right. so, most GM rear ends
either had a tag near rear diff, or stamped with a 7-digit
alpha/numeric on the back side of the tube/housing..
first 2 are ratio, second set is month/yr, third is day, last single is a/b/c shift
since chevy was cranking out more than buick/olds/pontiac,
it is probably from a caprice, monte, or impala..
another possible way to figure it out (if no tag; usually removed, and
if you cannot find stamping), is to take pic of the 10-bolt rear end and post..
pics:
https://imgur.com/a/RN2XIBJ
hope this helps..

[jleslein]

ctesla thanks for info- no luck with tag, will look closer for tube stamp. Drums on rear are huge- 12” ID on drums, the drums I bought for this car fit inside them!

[jleslein]

my photo won’t load “413 entity too large” but differential cover looks like far left from images in your link, with fill plug at about 3 o’clock and 10 bolts. Wish I could post pic, prolly my state of the fart I phone 6... and thanks again very much for taking the time to reply!

[Stealthee]

Upload your images to imgur and post them that way.

[ctesla]

Stealthee is right..
go to imgur 's website, drag your pic in, they'll give you link to post here..
odd's are rear end was swapped, so this might help:

https://www.chevydiy.com/history-and...differentials/

but, back in the day, many vehicles were custom ordered for police/taxi or heavy service use cars.. your Grand Am might have RPOs (maybe?) posted in glove box;
if so, here are some rear end codes to look for:

https://www.differentials.com/gm-axl...ication-codes/

but my old Laguna S3 chevelle did not have placard in glove box..
i found the build sheet stuffed in the coil springs under the back seat
when i pulled it once to retrieve a dang snap on wrench that fell behind
and under the cushion..

good luck.. and waiting for pics..

[xeroinfinity]

depending on when the car was made, you could have a previous or the next year rear end installed from the factory. GM was good about getting rid of crap they had laying around at the end of a year...or month or week.
Without seeing the yours its hard to guess exactly why its not correct per make model and year.
But this site might help you identify yours.

https://www.chevyhardcore.com/tech-s...10-bolt-chevy/