Upgrading Chevy 12 bolt to 9 inch

Discussion in 'Got gears?' started by TTNC, Oct 12, 2020.

  1. TTNC

    TTNC Well-Known Member

    I've got a Chevy 12 bolt in my car now. It uses the stock trailing arms and has Viking coil over shocks. The coil overs are bolted to the rear using UMI Performance brackets that bolt to the lower trailing arm mounts, and the coil overs bolt to those.

    The passenger side axle tube appears to be coming out of the differential (see photo). It was suggested to me to upgrade to one of the complete nine inch rears for 68-72 GM A bodies from Quick Performance rather than deal with a 50 year old rear end.

    I just got off the phone with QP and asked them if the lower trailing arm mounts on their nine inch would be similar enough to the ones on a stock 12 bolt that those UMI brackets could just be bolted right up. The guy didn't seem to think the trailing arm mounts would be exactly the same. Of course, that doesn't mean it WON'T work, it means we don't know.

    So I'm wondering how people have dealt with this, coil over shocks on a "bolt in" Ford nine inch in an A body.

    Alternatively, I've been asking around if anybody near me can weld the 12 bolt tube for me so I don't have to buy a rear end. The car is probably in the neighborhood of 475-500 hp and is street driven only, not raced. Any weld repair will need to make it hold up at least as well as when it was brand new.
     

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  2. I had a guy in Sutton weld my tubes to my 12 bolt center section a few years ago. Not sure if he's still doing it, but if you're interested I could track down his info.
     
  3. Bens99gtp

    Bens99gtp Well-Known Member

    I complete 9" with center is goingbto ve similar in price to a new 12 bolt rear end....both Brian and Jim can get mew moser housing......which are much better than the factory cases ever dreamed to be......I think from what I've seen they 1700-1800ish
    The 9" will more than likely require a driveshaft length change and ujoint

    I welded the tubes on my 8.5 with my mig welder........no issues......just cleaned it up and welded
     
    Last edited: Oct 12, 2020
  4. TrunkMonkey

    TrunkMonkey Well-Known Member

    Pinning the tubes can be done by removing the axles and carrier, leaving the pinion alone, (pack to keep the pinion area clean) drill, tap pin (bolt) the tubes, then clean housing and tubes, and reassemble with same shimming. Should be less than $100.
     
  5. BrianTrick

    BrianTrick Brian Trick

    I don’t see anything out of the ordinary with that picture. I don’t see any evidence that the tube is coming out. Oil whisking around it is very common. I was just working on one today that looks like that on both sides,and has never been beat on. It’s a 2:56 non-posi.
    As for the rear coilover kit,it will not fit the QP housing with the way they make their shock mounts. You would need to do some fabricating.
     
  6. BrianTrick

    BrianTrick Brian Trick

    Today’s project
     

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  7. TTNC

    TTNC Well-Known Member

    Sure, pass it along if it's not too much trouble. I talked to Mitchell Differential in Shrewsbury, they can probably fix it but they don't want me to bring them the whole car, they just want the rear end out of it.

    The QP nine inch complete assembly is around $1900 before any options.

    As far as welding the 12 bolt, I read on the Camaro forum that the best way is to use a nickel rod with a controlled preheating before and a controlled slow cooking after. This was for a 10 bolt, presumably the 12 is similar: https://nastyz28.com/threads/how-to-weld-axle-tubes-on-gm-8-5-10-bolt.18890/

    As I said to Paul above, I talked to a diff repair place near me. They could only give me a ballpark price without looking at it, maybe $250 or so if they don't have to replace the tube, maybe double that if they do.
     
  8. TTNC

    TTNC Well-Known Member

    I attached a zoomed in section of my first photo. It looks like the tube came out of the diff about 1/4" on the passenger side, like the O.D. of the tube looks smaller in that area because there's no paint there. Then it started to rust. That's a normal appearance?
     

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  9. TrunkMonkey

    TrunkMonkey Well-Known Member

    Gotcha. The amount I mentioned was you buying the kit and doing the work.
    Taking several measurements from the face of the flange on the housing where the tube inserts, to the flange on the backing plate on both sides, and post them.
    Might verify if it is actually moved or if it is just paint flaked away.

    (I have seen some things that I swear were one thing, only to find I had optical delusions. ) [​IMG]
     
  10. TTNC

    TTNC Well-Known Member

    There's a kit for this? Do you have a link?

    I like the measurement idea, are both sides supposed to be the same length?

    I suppose the other thing I could do is take the rear tires off and measure across the WMS, I'm seeing it should be 62.5". If the tube is coming out won't that dimension increase?
     
  11. TrunkMonkey

    TrunkMonkey Well-Known Member

  12. BrianTrick

    BrianTrick Brian Trick

    Totally normal.
    To weld that housing correctly,you want it in a jig,to make sure it is true and straight. If it’s crooked,you will weld it crooked and make it permanently crooked. You also want to heat the center before you weld it,to get the proper heat penetration. Otherwise you are wasting your time. Yes,you need to remove the rear from the car and pull the guts out of it,just like the housing I pictured above. That is what I am doing to it this week,along with cutting the ends off and welding Moser #7900 ends on for bolt-in axles.
     
  13. TTNC

    TTNC Well-Known Member

    What started this whole activity off is a wet spot under the diff on the floor in my garage. That weird spot on the passenger side tube is the only thing that was observed as questionable looking. I'm not sure what you mean by "Oil whisking around it" but if you mean it's normal for a slow leak to be there then thats encouraging, I guess? (If I don't have to replace or repair the housing). Can I get away with putting a bead of RTV around there?

    As a sanity check I will take some flange to flange measurements and throw them up here.

    It would be interesting to see a photo of what it looks like when a tube is really coming out, if someone has one.
     
  14. Bens99gtp

    Bens99gtp Well-Known Member

    These gm rear axles were known to seep oil through the rose bud weld area in the pumpkin........this is how the factory welded to tubes to the pumpkin
     

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  15. BrianTrick

    BrianTrick Brian Trick

    Sorry,it was a spelling error. It should have said wicking. The housing that I posted pictures of has wicking on both axle tubes,where they are pressed into the pumpkin. Some have none,but most of them will seep or wick somewhere. F the axle tube actually breaks loose from the pumpkin,it usually cracks or damages that area of the pumpkin. The axle tube will usually turn or rotate position with the torque of the car.
    Wheel-hop can cause the tubes to break loose from the pumpkin and even break the pumpkin itself. This was a common casualty of trying to run air shocks,bad bushings,bad geometry,etc.
     
  16. TTNC

    TTNC Well-Known Member

    So then what is the appropriate fix? Should it be welded or I can seal it with something?
     
  17. B-rock

    B-rock Well-Known Member

    I have a Quick performance 9" rear in my 65. for the most part it was a direct bolt in. messed around with the parking brake cables a bit, but it went right in. I also had a chevy 12 bolt, I sold that and paid for half of the cost of the 9". to me it was a great upgrade. everything was better all the way around. yup with the disk brake upgrade.
     
    Bygblok and Lucy Fair like this.
  18. BrianTrick

    BrianTrick Brian Trick

    The OP has a rear coilover kit that has brackets that won’t directly bolt onto the QP shock mounts.
    There is a lot of misleading info about a 9”,making people think they have bought a cure-all. Your base-model,budget 9” that most companies will try and market due to price does not have the better parts to really make it worth the purchase. You can have a $500.00 center,or an $1800.00 center. There is a difference. Some goes for the housings and shafts.
    As for the leaking 12-bolt housing: using a sealer might work for awhile,but it will find its way back out. Welding it will plug it 100%.
     
  19. TTNC

    TTNC Well-Known Member

    The coil over brackets I have bolt to the lower trailing arm mounts, not the shock mounts. The shock mounts on my 12 bolt are gone. If I go the nine inch route I was asking if anybody knew if the UMI coil over brackets would bolt up to the nine inch lower trailing arm mounts. This would probably mean cutting the shock mounts off the nine inch. QP couldn't tell me either way.

    I will see if the diff repair shop near me will take a look at it and see what kind of a job they think it will be.
     
  20. TTNC

    TTNC Well-Known Member

    UMI coil over bracket bolted to lower trailing arm mount
     

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