How do you remove a rusted cast iron waste pipe without damaging it or the threads?

Mix up a glob of JB Weld and then screw the cap in. Once that sets, it is never coming out. The other answer would be to get a rubber connector boot bigger than the pip and connect a new piece of PVC which you could cap. vbd Purchase a No-hub clamp at a Plumbing supply house, along with a cap, or cleanout plug. The No hub clamp slips over the damaged threads, and connects the new fitting. Tighten the screws. Done! There is a plug availible, rubber with a washer each side and a bolt running thru it with a wing nut on the top side. Simple insert into the hole and tighten the wingnut. This compresses the rubber causing it to swell out and plugging the hole.