It would appear that to get a nice seating that the old threading pretty much HAS to be missing or it won't seat in all the way. In addition it would appear that for some BB shells that a drum sander to ease the stripped threading a little to allow proper seating of the flanges may be called for. A bit of a pain but it's still an option that will keep an old frame on the road instead of resulting in scrapping it for ruined threads.