I'm replacing the bathroom sink in a vintage building from the 30s. The cast iron rough-in pipe no longer has threads. The prior coupler was a 1 1/2" threaded pvc fitting, but it popped right out as it had nothing to catch on. I attempet to shove a 1 1/2" galanized nipple in there thinking it would catch, but it did not. I would try to re-tap the pipe, but a pipe tap that big cost too much. I saw a product called a Fernco hub donut that looks like it might work, but I may have trouble finding the right size. The dimensions of the rough-in are OD 2 1/2" and ID 1 7/8". It tapers in. Any suggestions on how I could connect my sink drain to this old rough-in pipe would be appreciated. Thanks.
I rented a 1.5" pipe tap, but the drain was too deteriorated to make threads. So, I cleaned the inside surface of the drain with a wire brush rotary bit. Then I took a short section of 1.5" PVC pipe and sanded down the outside surface of one end until it fit snugly within the iron pipe. I coated both surfaces with Amazing Goop Plumbing sealant and shoved the PVC into the iron pipe, tapping it in with a hammer. I put some additional sealant around the edges where the two pipes met, both inside and outside of the iron pipe. I waited a bit while it set, then I attached a PVC joint trap adapter to the PVC pipe with PVC cement. I waited about a day to hook up my p-trap. It's been over a week with regular usage, and I haven't seen any leaking. Thanks for all the suggestions!