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Originally Posted by 87Fbird
I don't know if anyone's ever done this before. I know people swap 12 bolt (C) rears into their 3rd gens all the time but can you just as easily put a 12 bolt truck rear in as well? I got one sitting in my dads garage and it has no purpose. I'll have to cut it down 3 inches and swap mounting hardware but what do i do about the tourque arm? Where would it mount up to? Forgive me if this is a stupid question but i just bought my 87 Firebird last month and i never dealt with F bodies before. Does anyone have any tips?
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Not worth it because you would have to do some cobbling.
1.) It won't have the mounting stuff you need. I believe the torque arm mount is cast into the center section of the 3rd and 4th gen rears so it's not easily added. You would have to preheat the center section before welding and take the proper steps in welding steel to cast iron (assuming your TQ arm mount would be steel). Going to a 4 link or ladder bar setup is doable but costly.
2.) Narrowing the axle tubes would also require you to narrow the axle shafts, which pretty much means you'd have to buy new ones. Narrowing axle tubes is also risky to do without a jig to keep everything square.
3.) Gearsets are not the same. The truck pinion stem diameter is only 1.437 vs. 1.625 on the car unit so you could not use passenger car gears. The passenger 12 bolt is stronger for this reason.
IMO, not worth it. The crappy part is that no 3rd or 4th gen came with a 12 bolt stock (I think the Dana 44 is the best they got, and that's good but really rare) so your best bet would be to search online for a used 12 bolt or pony up for a new rear (12 bolt, 9", or S60). I'd find out what gears and differential are in it and see if you can sell it to get money towards a new rear or other modificiations. You might be able to re-use the posi but if I were you I'd only re-use it if it's an Eaton.