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Old 07-23-2005, 10:59 PM
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12-Bolt Posi GM

I have a 12-bolt GM Posi rear axle and I was told it came off a Firebird. Is there a code I can tell which year it came off from and maybe the gear ratio? Thanks...

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Old 07-23-2005, 11:09 PM
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Gears

If you take the back cover off the housing and on the ring gear you will find #'s stamped in,if I remember right you will find a # like 41-9 or 45-8,,well you take 41 x 9 is the ratio,the GM # stamped in you would have to call GM dealers and have them look it up.
Now it has been awhile since I played with these but when I used to drag race I think this is how it was done..
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Old 07-23-2005, 11:15 PM
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rear end code

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Originally Posted by Photo Car Show Team
If you take the back cover off the housing and on the ring gear you will find #'s stamped in,if I remember right you will find a # like 41-9 or 45-8,,well you take 41 x 9 is the ratio,the GM # stamped in you would have to call GM dealers and have them look it up.
Now it has been awhile since I played with these but when I used to drag race I think this is how it was done..

cool, thanks! Is there also a code on the axle tube that might help? Maybe give me the year of the car?
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Old 07-23-2005, 11:42 PM
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The code is usually found on the right axle tube, a few inches outboard of the center section, usually on the front, maybe toward the top. Scrape off the crud & you'll find it. It will only tell your what ratio came in it, not what it might have been changed to over the last 35 years.

When you calculate the ratio, divide the small number by the big one. In other words if you see 41 9, divide 41 by 9 to get the ratio.

41 / 9 = 4.56
42 / 13 = 3.23

Other numbers on the ring gear will be a part number (6 to 8 digits) and a date code (11 69 would mean it was made in November or 1969, etc.).

Measuring the distance between the leaf spring perches (center to center) might also help ID what year car it fits.
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