12 bolt to 10 bolt swap with OD

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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, 12 bolt to 10 bolt swap with OD in Past Cadillac Vehicle Discussion; hello I have a 386/th400 12 bolt 81 Fleetwood. the rear suspension is very worn out and the axles are ...
  1. #1
    81Brownie's Avatar
    81Brownie is offline Cadillac Owners Fanatic
    Automobile(s): 81, 87 and 95 Fleetwoods, 89 Brougham
    Join Date
    May 2013
    Southeast WA

    12 bolt to 10 bolt swap with OD

    I have a 386/th400 12 bolt 81 Fleetwood.
    the rear suspension is very worn out and the axles are worn. it dogtracks bad!
    Im going to rebuild the old 12 bolt, so this will be a temporary rear end.
    yea I know im going with something weaker but I want to get the most out of the 368 while I find a 500 to touch up. plus maybe ill find a nice 12 bolt there waiting for me.

    this weekend im going to the junkyard. I plan on getting the 10 bolt axle, driveshaft, th200 Overdrive trans, control arms and maybe the leveling shocks. I found a 65k mile flipped 85 with the 4.1 L and that's what im planning to attack soon.

    this SHOULD all bolt up nice and easy right everyone? I prefer the OG driveline and it will be rebuilt and installed in the future. this is just temporary! im hoping I don't need to cut up anything in the car since I will be installing a 500 and everything else back in there.

    well, im hoping to end up with a 368/OD Caddy that drives straight and gives me better MPGs! plus I can get all this for about 150-180 bucks easily! any help would be appreciated


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  3. #2
    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
    Automobile(s): 81 FWB D'Elegance coupe
    Join Date
    Sep 2006
    Ontario, Canada

    Re: 12 bolt to 10 bolt swap with OD

    Don't forget that you need to run a TV cable for the OD trans. Also, you need to do some math. 4100 cars came with 3.42 gears. Even with OD, you might actually have higher cruise RPM than your TH400 combo. I forget what the stock gears in the 81 368 is (I have one, but it is mid/low 2.xx).

    It's a lot easier to do a set of axles/bearings and control arm bushings than a rear swap and a custom trans swap... especially for two marginal pieces (the weak 200-4R/7.5" Chev rear)

  4. #3
    cadillac kevin's Avatar
    cadillac kevin is offline Cadillac Owners Master
    Automobile(s): chevy 350 powered 86 FWB, 00 safari h.t. 66 toro, 83 lesabre
    Join Date
    Dec 2010
    St. Louis, MO
    I would not put a stock 200r4 behind a caddy big block. The torque of the engine will most likely destroy it.

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  6. #4
    The Ape Man's Avatar
    The Ape Man is offline Cadillac Owners Connoisseur
    Automobile(s): '80 Fleetwood Coupe, 1994 and 1995 Mercedes 140 Coupe
    Join Date
    Feb 2005
    Southern NJ

    Re: 12 bolt to 10 bolt swap with OD

    The overdrive will land up being almost exactly the same as the 2.41:1 you are removing : )

    May get some additional economy from the lockup if it is working.

    One needs to have all their ducks in a row to run a 200-4R behind any torque. In the end it might be better to do this another way.

    the axle itself should not be causing the car to track as you say. Sounds like seriously bent or even broken trailing arms and/or frame component problems.

    Spray that thang with brake fluid or yer favorite penetrating oil ahead of work. The bolts will thank you on the way out.

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