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Old 06-18-2012, 01:32 PM
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whats up with my 12bolt

i believe i have a 12 bolt chevy rear end but i came across somthing wierd i bought a new cover for it but bolt holes didnt line up then i looked at the old aftermarket cover and someone reamed the holes out to make it fit has anyone heard of such a thing i really like to know whats going on here i also looked for some #s but didnt find any on the axle housing and just a couple on the carrier help please

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Old 06-20-2012, 07:31 AM
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What is it supposed to be out of? GM made more than one 12 bolt rear. If it is the real 12 bolt (that we all know and love) it will have an 8 and 7/8 diameter ring gear. I would just go by the local NAPA store and have them look up a cover gasket for a 70 chevelle with 12 bolts. You didn't mention mounting dimensions. DO NOT ream out the holes. You have either go the wrong rear or the wrong gasket. Use Google image to compare pics of it.
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Old 06-20-2012, 10:00 AM
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this is in a 1970 el camino i did have a gasket i know that theses rearends will fit alot of applications i dont believe you can take any 12 bolt rearend unless it from a A bodyan has the ears on the housing for the upper control arms.Now with that said ive read that a 12bolt is a 12 bolt no matter what excepet for some olds rearend thats really a 10 bolt internerally so this is what i got 1970 elcamino with a rerend that has 12bolts for the cover no numbers stamped on the axle tubes except T3 and on the housing ifound one set of numbers 402227 another set thats looks like its on a tab thats 57 a sideways U and a circle of dots with a arrow pointing to what would be 9 oclock it is a posi with 373 gears so any more info would be appreciated DR327 i appreciate your response i would check the ring but i have the old cover back on with oil in and back in the car right now. I will use it till i figure this out or get a new one
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Old 06-20-2012, 10:15 AM
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There are 4 GM rears with 12 bolts.
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Old 06-20-2012, 11:19 AM
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OK if there is 4 gm 12 bolts and do all 4 have different bolt pattern for the cover and out of those 4 would they all bolt into a 1970 el camino and if so what information do i need to locate the correct cover because the 1970 chevelle or A body cover didnt fit what are my other choices .One quick question will a truck 12bolt fit in my 1970 elcamino an will the cover have a different bolt pattern

Last edited by kpj38@hotmail.com; 06-20-2012 at 11:28 AM.
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Old 06-20-2012, 11:39 AM
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There is
12 Bolt Truck
12 Bolt Car
12 Bolts Olds
12 Bolt Big passenger car
All have unique bolt patterns, covers and gaskets.

The only one that would be a direst bolt in would be either the 12BC or the 12BOlds.
The big passenger axle might go in there but would be longer than the factory axle.
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Old 06-20-2012, 02:18 PM
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I'm thinkin' the cover that was on it was wrong and that is why the holes were reamed. You've probably got the real deal. You can probably go to one of the many Chevelle sites and use your cast# to find out for sure. Imsport is right and it's pretty easy to count the ring gear bolts. A lot of people think 10 and 12 bolt refers to the cover, but it doesn't. It refers to the number of ring gear bolts. I would be more apt to believe your dealin' with the wrong cover and not the wrong axle.
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Old 06-21-2012, 02:46 AM
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thanks alot you guys i learn a little something today now since i tried the chevelle el camino gasket an this is how i came to this question in the first place really my only other 2 option is the full size gm or the olds Now when we are talking about the olds rearend is this that funky one and full size gm would be impala also is there some good place to find some of this info on the net thanks again
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Old 06-21-2012, 09:14 AM
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If its an Olds rear, it will not have c-clips in the diff side gears to retain the axles.
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Old 06-21-2012, 12:11 PM
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http://www.drivetrain.com/parts_cata...Specifications

Your diff look like this? That's a normal 12 bolt car

http://www.drivetrain.com/parts_cata...d_pinions.html

12 bolt truck.

That's a pretty good site to ID diff's by the rear cover.
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