A wide crankset was needed to accommodate my new centrifugal clutch. I also wanted to run a multigear chain in my 1976 Western Flyer, so I had to install an MTB bottom bracket adaptor and 3 piece crank. I already had a singlespeed kit on my hyperglide cassette rear hub. I bought the TruVativ bottom bracket adaptor. It slides into your bottom bracket tube and the two sides are held together by 3 long, thin screws. It looked easy.
After removing my old one piece crankset, I found the bottom bracket tube in my frame to be full of obstructions. Frame tubes potruded into the bottom bracket as much as a half inch. The bottom bracket tube contained another steel tube within. A dremel tool worked too slow. I had to use a hacksaw and rasp to clear out the bottom bracket tube enough so that the three screws from the adaptor could pass all the way through.
The final problem was that the cups in the bottom bracket adaptor could not be slid all the way in due to the second, concentric steel tube within my bottom bracket, but I could seat and tighten the three screws. When I installed bearing cups from the MTB bottom bracket, it threaded in too far to attach the lock ring before the bearings were tight enough. I threaded in a second bearing cup (I had a few laying around) to use as a lock nut and was able to thread the lock ring on it.
The adaptor from TruVative worked fine. The sloppy metal work done in the construction of my old frame made it an ordeal; requiring metal removal and extra bottom bracket parts. See pic.