I was wondering if a 10 bolt axle housing itself is the same between drum and dic for pre 89 apps. I have an 85 trans am with discs, and can get a drum axle locally. Can't I swap axle shafts and backing plates/calipers rotors? Just want to make sure. Any help much appreciated

 

Nope, not the same.

Disc ones have a small, almost symmetrical (emphasis on "almost") flange w 4 bolts.

Drum ones have a flange that's smaller at the bottom, has only 2 bolts (which are in different places than any of the above 4), and a large thing that sticks upward, that the slave cyl goes through and attaches to.

Nowhere near a direct swap. But, if you can drill WITH PRECISION, it is said that it can be done. Never done it myself; I always have just hunted down the right part.

Might want to consider 4th gen rears instead.

 

Check out www.autoweldchassis.com

 

Would these instructions for drum to LS1 brake swap work for the 85 discs, too?

 

I have converted a few drum rears to disc, it's not hard at all.
Have to cut off the top of the drum rear flange, drill some holes and use disc rear axles or have the drum axle flange turned down a bit so the rotor sits flat on the axle.
Easy.