Can you ID this 12 bolt from a '71 Skylark?

Discussion in 'Got gears?' started by Torque2, Jul 27, 2005.

  1. Torque2

    Torque2 Well-Known Member

    Hey, I pulled a 12 bolt posi rear from a boneyard Skylark a few years back and am now interested in putting it in my '70 GS. It has 3.08 gears and I have had no luck matching up codes from a bunch of Chevy/BOP sites.
    Any ideas?

    Axle Tube(passenger side)= CX 11 28 B
    Diff. Housing(driver side lower rear)=J 307
    Diff. Housing(driver side lower front)= NF(on top) X3917124(on bottom)

    Thanks!

    Tim
     
  2. GStage1

    GStage1 Always looking for parts!

    If the cover is round, it is an Olds 12 bolt and worth nothing.

    If the cover is oval, you have a Chevy 12.

    A pic would be a big help.
     
  3. Torque2

    Torque2 Well-Known Member

    Here you go. Not sure how oval oval would be. I hope the pics make it clear.
     

    Attached Files:

  4. Lon Bauer

    Lon Bauer Well-Known Member

    The eye brow in the back cover makes it a Chevy 12 bolt.....right George???

    Lon
     
  5. GStage1

    GStage1 Always looking for parts!

    Lon wins the prize for the day!

    Chevy 12 bolt

    The car you removed it from probably was a "MADE IN CANADA" (appears on the cowl tag) car.

    When you install that rear in your car, you will need NEAPCO 3-3130 for the rear u-joint (it is an offset u-joint) and costs about $25.
     
  6. 10sec 455

    10sec 455 Well-Known Member

    You will only need this ujoint if it came out of a chevelle. If it came out of a Canadian Buick no conversion ujoint is needed.
     
  7. Torque2

    Torque2 Well-Known Member

    Great news! Especially since I only paid $65 for it. Would I need to change anything if I am putting it behind a 455 w/ a TH400? That is what I have in my '70 GS 350 w/ a stock peg leg. I'm thinking it may require a change in the length of the driveline.
    Thanks for clearing up the Olds/Chevy issue!

    Tim
     
  8. GStage1

    GStage1 Always looking for parts!

    You will need to shorten your driveshaft. If you get back to the yard where you got the rear, you can verify if it was a Canadian built car. Since you have the rear out, get a posi unit for it. Otherwise, you will just burn off the right rear tire.
     
  9. monzaz

    monzaz Jim

    Drive shaft

    Double check it first before doing the drive shaft thing. Could be pretty close. Just make sure before spending and cutting. Jim
     
  10. Torque2

    Torque2 Well-Known Member

    I'm actually headed back to that yard tomorrow, but the car is long gone.
    But then again, who knows what else I might find. I found a '73 Stage 1 GS there about 10 years ago.
    So I need to bolt it in and take a measurement for the shaft, right? How far onto the output shaft is considered acceptable for the front yoke? I know a little room helps to separate the driveline from the rear end, but where do you draw the line?
    Also, the 12 bolt is a posi unit, but I don't think 3.08 gears are going to work well with a Hemi-killer cam and a 3,500 stall 10" converter. I'm told I need to be in the 3.55-3.73 range. This car won't be on the highway much.

    Thanks,
    Tim
     
  11. monzaz

    monzaz Jim

    code

    I think cx is 3.07 code if I am not mistaken...Not a huge difference. Just may have a different pinion ring number combo to get that number 3.07 3.08 all the same to me. Jim
     
  12. Torque2

    Torque2 Well-Known Member

    I just pulled the cover and counted teeth - 43 / 13 = 3.07 You are correct sir! Thanks.

    Tim
     
  13. GStage1

    GStage1 Always looking for parts!

    Typically you want 1-1.5" of "slip" on the slip yoke. Basically, slide in your trans yoke all the way. Measure from center of u-joint cap opening to center of u-joint cap opening on rear then subtract 1-1.5" You should be right around 55"
     
  14. oPh

    oPh Well-Known Member

    Going from 8.2 10 bolt Buick to an 8.5, or 12 bolt Chevy to 8.5 corporate a-body rear, yes, the factory length 8.2 driveshaft should need to be shortened. Going from any 8.2 10 bolt to 12 bolt Chevy, the factory 8.2 driveshaft should not need to be shortened.

    Only case, I've ever run across, where a factory 8.2 driveshaft might need to be shortened, is if someone has removed factory 12 bolt 1310 or 3R u-joint flange & replaced it with the 8.5 yoke with 3R flange. Dimensionally the 8.5 flange sticks out further. When building 12 bolt rear needing a 3R flange, I don't resort to the 8.5 flange, it creates just such problems.
    :3gears:
    Roger

    Editted for clarity
     
    Last edited: Aug 2, 2005
  15. GStage1

    GStage1 Always looking for parts!

    I think he is installing a TH400 with the 455.
     
  16. monzaz

    monzaz Jim

    ratio 3.07

    NO if those are the correct number from the ring gear I am wrong. 43 and 13 is a 3.31 ratio Make sure your count is correct and or check the tube again for the correct letter code OR someone has changed the gears. Or your count is wrong??? Please double check everything. The outside edge of the ring gear is where the stamp will be for the ring and pinion gearing. If this is where you found the number then just go with that and don't worry about the axle tube...obviously. The exact reason for ALWAYS opening and inspecting the parts before assuming it is something. Great example. Jim
     
  17. monzaz

    monzaz Jim

    ratio

    The ratio should be something like 40 on the ring and 13 on the pinion. Ok? If it is a 3.07 3.08 rear. Jim
     
  18. Torque2

    Torque2 Well-Known Member

    Yikes!
    Always double check your math! Good thing I'm not a math teacher any more. The tooth count is correct: 43/13. I found it by counting teeth and by the pinion gear number stamp. It is a 3.31 ratio. Both gears have the same yellow "B 45" stamp on them, but not sure how you would know if they are original.
    I have a 455 and TH400 sitting in my GS350 right now. So the driveline has already been shortened for that application with the existing 8.2 rear.
    Now I'm wondering if it is too short for the 12 bolt.
     
  19. monzaz

    monzaz Jim

    Choices

    Well I guess you have 2 choices...lol. Get a 8.5 to match the drive shaft or get a drive shaft to match your 12 bolt...lol Good luck with it. If you want to ever off those gears give me a email and I will make an offer with pictures from you. Jim :)
     
  20. oPh

    oPh Well-Known Member

    Torque2, no need to shorten driveshaft, as you already have shorttail Turbo 400 installed. Definitely pull the 12 bolt axles out & ck the condition of the axles where the bearing rides. Pull the rear cover off & flush the 80-90. Everything appears fine, the swap is a go.

    Installing the 12 bolt in your Buick, can speed things up considerably by pulling drums & axles from your 8.2 10 bolt, then removing control arm & shock bolt nuts. Bend the little line tabs up & leave the 8.2's brake lines hooked up with backing plates hanging from coat hangers or small bungees. Drag 8.2 rear from under car. Next, simply roll the 12 bolt rear, with no axles & backing plates , balanced on good sized floor jack , under the rear of car, lift up, bolt up, & use your current loaded backing plates & intact brake lines. With your orig rear brakes in great shape, have no brakes to rebuild, no brakes to bleed. If bolts are not rusty & everything is ready to go, should be able to this on garage floor w/o airtools in under two hours.

    With the good luck you have (finding the 12 bolt with 3R flange is a godsend), you should have no problems.
    :3gears:
    Roger
     

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