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12 Bolt - Identification - not Centered - assistance


snomobeelr

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Hello ,     I've had this 12 bolt rear end in the car since early 90's. (91/92)  I bought the Housing and axles  and the carrier and gears  separately  and had it built by someone I was told they knew what they were doing. I was told it was a 3 series carrier and I am running a 3.73 Gear.   It may be built correctly I do not know that for sure.. When the  frame was off the car we measured it and it measured square.   When I was looking for wheels I was having trouble because the Driver side seemed inboard 3/8 to 1/4  and the Passenger side seemed outboard 3/8 to 1/4 . I never noticed or paid attention before. So I pulled the cover again and It appears the carrier was shimmed to one side in the 12 bolt.  Drivers side is shimmed and in passenger side bearing sticks out enough I can read Timken on it.   I am  trying to figure out if it is actually a Chevrolet 12 Bolt carrier and if it is put together correctly . 

    Too much reading and I am wondering if this is an Oldsmobile Housing?  I have added some pictures with the cover off.   I didn't find any numbers on the tubes but there are some on the housing. The Housing has two Straight ribs on the front. The Numbers I found on the housing that faces the engine were GM8 on the Left and and  "3917124"  NF" on the right side. On the side of the 12 bolt housing that faces the rear bumper I found  L157  on the left site and what appears to be CON 1 on the right side.  I can not find any info on the  "CON 1"  stamping.  Below is what I found on the rest. 

http://www.myss396.com/ChevelleInfo/Differentials/Differentials.html

L/15/7

..........................................

L - December

15 = 15th Day

7 = Production Year?   1967

*******************************************

 12 Bolt Housings

Casting         Year

3917124NF          1967 (Late)

3917124NF          1968

3917124NF          1969 (Early)

12  BoltRear End 1.jpg

12  BoltRear End  2.jpg

12  BoltRear End  3.jpg

12  BoltRear End 4.jpg

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Hey snomobeelr,

You have the Chevy style 12 bolt. You are not correct in your rear not being centered. As weird as this sounds, it's "normal". There's nothing bent or twisted on your car. 

I know for sure Chevelle's, LeMans and gtos have the same issue. For whatever reason, (I'm sure someone has an answer) the GM cars are not perfectly centered on the frame. 

That is why usually one tire rubs on the inside or outside of the rear fender bender when the wrong  offset wheel or big tires are used. 

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Thanks Guys. So it is a Chevy 12 Bolt. Here are some more pictures. The ring gear has a big spacer. I guess I need to measure to verify this is a 3 series carrier.   I do not understand about the carrier not being centered. If this is normal than thats fine but seems like it should be centered.  The ring gear has a spacer and I wonder if maybe it was supposed to be a different ring and pinion..  Here are some more pic's. Trying to look up the part specs by the numbers..  

12 BOLT .jpg

12 Bolt 2.jpg

`12 bolt 3.jpg

12 bolt 4.jpg

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By all indications it's a 2-series carrier. The pinion gear is going to pretty much be a 'fixed' location (proper depth required though for proper tooth engagement with ring gear). The carrier assembly is going to be set left/right based on that to get proper tooth engagement & backlash. With that spacer and 2-series carrier, it's pretty much going to land where it is with shims to get set up for an acceptable pattern. Most sources will agree though, that the spacer & the ring gear bolts in a double shear situation aren't ideal for any high horsepower use.

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On 2/13/2019 at 12:39 PM, jft69z said:

Looks like a 2-series carrier with a ring gear spacer?

 

The diagram in the below link shows where to measure:

https://www.motorstate.com/TheVenerable12-BoltChevrolet

Check out the interesting link jft69f posted. Looks like you have a 2 series carrier with a spacer for 3 series gears. Not that it's wrong, just that's how it was done. Cheaper than buying a 3series posi carrier. 

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Thanks Guys thats what I wanted to know, if I have a two series carrier and thats why the shim is in there and possibly why it is not centered. I was told it was a 3 series carrier years ago. I may leave it for now , I needed to know what I actually have.  I want it to be right and I may change it to a 3 series carrier if I can find someone who actually knows how to do it later down the line.  Or attempt it myself. I am not sure about that though.  

Thanks agian 

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