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  1. #1
    Join Date
    Jul 2008

    Default A- body rear into g-body

    Has anyone done a 68-72 chevelle rear swap into a g-body. I know the chevelle rear is about 4" longer axle to axle, the LCA are mounted the same but the uppers are not. I think umi performance makes uppers, want to run a 8" wheel, what backspace would i need, its a 8.5 rebuilt with a posi and 3:73's for $400. Thanks

    68 Chevelle 496 tko 600 12bolt
    81 Malibu 350 th350

  2. #2
    Join Date
    Mar 2008
    Sparta, TN


    GM 12 bolt- stock specs: 8.875" ring gear, 1.625" dia. pinion shaft, 30 spline axles.

    Swapping a 12 bolt into a G-body is probably the most difficult conversion to complete. The problem lies with the difference in control arm mount angles. You can force the arms to bolt up, but suspension movement will bind up severely. The housings are also different widths which can be corrected with wheel offset (also commonly called backspacing). The housing can also be cut to maintain stock width. The 12 bolt is the route I chose when building my 79 Malibu because there were no conversion 9" housings available when I began my project. I also prefer to keep my GM car ALL GM. I have to admit that the procedure I am about to explain is also one of the most expensive to do.

    The first thing that needs to be done is to fabricate a jig fixture to locate the lower control arm mounts and stock pinion location. At this point you need to remove the lower control arm mounts and spring pads from the 12 bolt housing. You also need to remove the same pieces from the 7.5 10 bolt. Next, install the 12 bolt housing in the jig and also 10 bolt control arm mounts and spring pads. After double checking your measurements, you can weld everything up. Now the lower control arms (of the 12 bolt) will be at the same angle as the 10 bolt was. The upper control arms are a different story. Since they are part of the cast center section, they cannot be modified. The solution is to use an upper control arm that has spherical rod ends to allow for the mis-alignment of the angles. At this time it would be wise to add "no-hop" bars to allow adjusting of the instant center. The control arms and no-hop bars I run are available through Art Morrison Enterprises. The rod end also allows for pinion angle adjustments, which comes in handy when you need to compensate for track changes. Being able to make such adjustments will make your car be able to hook even when track conditions change. Very important in my opinion!

    NOTE: Careful measurements will need to be taken to allow for proper wheel/tire fitment. The 12 bolt housing is wider than the stock 7.5 rear. I am still looking for the stock 7.5 rear width (drum-to-drum), but will provide the 12 bolts width for your comparison. The 1964-67 Chevelle housings measure 60.5 inches drum-to-drum. The 1968-72 housing measures 2 inches wider which would be 62.5 inches drum-to-drum.

    And yes, UMI makes adjustable upper control arms.

    My main point to quote that was the measurement drum to drum I guess.

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