#1  
Old 11-12-2022, 09:13 AM
dv657172 dv657172 is offline
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Default Differential ID - 71 GTO

Originally posted in 71 - 77 Ventura section in error

71 GTO

Numbers matching nearly top to bottom but....rear end looks to be a 12 bolt (posi) and PHS docs dont show a Saf-T-Trak option. Plus car is a 400 with AC which would come standard with a 10 bolt 3.23.

I have looked on front of axle tube on passenger side but cant find a code and differential cover doesnt look like the 12 bolt in my 72 GTO or in the service manual (see pic - note pic is upside down)

Any idea whether this is just an aftermarket cover or should I look somewhere else for a code or something to id this thing?

I have owned it since 2009 and have previous owners receipts for parts and service back to mid 1980s and nothing shows any rear end work

I know its not likely the correct rear, just trying to id it. Feels like around a 3.23 gear
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  #2  
Old 11-12-2022, 09:18 AM
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Looks like you have the Olds 12 bolt, O type.


Last edited by 400 Lemans; 11-12-2022 at 09:23 AM.
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Old 11-12-2022, 09:25 AM
dv657172 dv657172 is offline
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Is that good, bad or indifferent vs. 12 bolt found in GTOs?

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Old 11-12-2022, 09:34 AM
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http://jdrace.com/content/diff_id/12bABodyOlds.phpNot as strong as Chevy or Pontiac 12 bolts. It has 10 bolt internals and parts are more expensive and a little harder to get. But not the worst one to have. The 442,s had them and they held up alright.

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Old 11-12-2022, 09:48 AM
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Thanks 400 Lemans, you have been very helpful!

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Old 11-15-2022, 12:20 AM
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Yeah, Olds "O" type. 10 bolt rear with a 12 bolt cover. My 68 had one under it from the time I got it until just this past summer when I changed it out. I never could make sense of the 'code' on it...It has what appears to be either TI or IL but neither one of those exist in any documentation that I could find.

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Last edited by Greg Reid; 11-15-2022 at 12:43 AM.
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Old 11-15-2022, 11:10 AM
dv657172 dv657172 is offline
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Greg

Mine has a similar stamping although not as easy to read as yours or as clean, lol.

It looked to me like T 4 but I am not sure either

DV
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Old 11-15-2022, 11:29 AM
Mr. G Mr. G is offline
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T4 is the code for a 1967 O type rear end from a Vista Cruiser. 3:08 open rear end.

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Old 11-15-2022, 12:14 PM
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Hmm...Well I am either reading it incorrectly or its been modified as its definitely a posi rear and feels a bit taller than a 3.08 although I am not an expert

Do you think it looks like a "T 4"?

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Old 11-15-2022, 12:14 PM
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Also where did you get info from for future reference please?

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Old 11-15-2022, 01:44 PM
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I found a thread on the Classic Oldsmobile forum where codes were discussed. It would be best to wire brush the axle tube to be sure what the code is and if there are other indicators of what the rear end is. I have an old book called 442 by the numbers and it shows most codes found on 442 rear ends but only one is a T4 and it is a Chevy type 12 bolt rear end. Clean up the rear and see what other codes there are. Some should be on the center section also. A date code will be needed also as many codes where repeated through the years, but the date code will narrow it down better.

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Old 11-15-2022, 03:24 PM
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Mine is a 2.78 if l remember correctly. I've looked mine over thoroughly and I still have it sitting in my storage room but never found anything more than what you see in the picture I posted above. I do believe that ol' Pinion Head told me that something about it indicated to him that it was from a Vista Cruiser. Spring perches perhaps? Can't remember.
I tried to give mine away here in the forums if someone would come and pick it up but no interest at all.
I'm having some work done on the house after New Years and there will be a drag off. It's going to wind up in it first thing.

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Old 11-15-2022, 04:11 PM
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Well I did some better cleaning and the code appears to now be "T M"

New picture attached but I havent found any date codes but I havent given up yet.

Do you have " T M" in your book?
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Old 11-15-2022, 05:01 PM
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https://www.v8buick.com/index.php?th...t-1970.108431/ There claiming a TM to be 3.42 posi. Saw another older thread where a 12 bolt O-type was for sale and it was a 3.42 posi also.

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Old 11-15-2022, 05:03 PM
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Here.https://classicoldsmobile.com/forums...tm-code-52235/

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Old 11-15-2022, 05:08 PM
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Thanks 400 LeMans.

That would seem to be the case. Its definitely a posi and that gear ratio seems about in line with how it feels. Appreciate the info and websites!

DV

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Old 11-15-2022, 05:15 PM
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Yes. TM is a 3:42 posi . The same code was used on the 3:42 posi rears from'67-'72. That is according to my book.

  #18  
Old 11-15-2022, 06:12 PM
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Thanks Mr. G & Lemans 400 for all the info.

Seems like an ok rear end to have, better than an open 3.23 that it likely had from the factory given the ratio and posi.

DV

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