The front subframe of E24 and E28 cars is prone to breaking where the steering box is mounted. The first time it cracked, my local shop was able to weld up the cracks with the subframe still in the car. One year later while swapping out my track brakes and reinstalling my street brakes after a track event, I noticed a crack at the steering box mount. Upon closer examination, I found a total of three cracks! The cracks were not in an easy to weld location so I decided to drop the subframe. I also wanted to find a new subframe because mine had been repaired once already. I got a serious case of "while-you-are-at-it" and decided to replace the steering box, because my 218k mile box was a little vague on center. I located a low milage (<100k miles!) subframe and steering box from a car that had been damaged in the rear. I machined a piece of steel angle to box off the front part of the inboard mount and also to reinforce the top of the mount. A couple beers got a friend of a friend to MIG weld everything together. Both my old and "new" subframes showed some corrosion on the driver side which was probably due to years worth of brake fluid spills, so I cleaned the new one with POR-15 Metal-Ready and painted with POR-15. The new subframe went in easily, and five hours after rolling the car into the garage, it rolled out under power.
Here is the order of disassembly as best as I can remember. If you are not replacing the steering box, the job is even easier.
"Installation is reverse of removal." Don't forget to fill your power steering/brake system with ATF! Check out the pictures of the repaired unpainted subframe below. The white residue was left behind by the Metal-Ready.