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.

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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!

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  • Thanks for the update and for accepting the answer that suggested it.
    – FreeMan
    Commented Jan 14, 2022 at 15:18

1 Answer 1


Pipe taps appropriately sized are available for hire, along with something to drive them. A friendly plumber may well have a fitting one, and would do the job for a drink? Otherwise, I'd go with pvc pipe, roughed up outside, and two part epoxied in, leaving a usable stub. Locality would help, maybe?

  • I'd go silicone glue rather than epoxy - bit of flex to it - like loctiteproducts.com/en/products/specialty-products/specialty/…
    – Tetsujin
    Commented Jan 3, 2022 at 17:08
  • @Tetsujin - that may help, although the cast iron it'll go into never had any flex!
    – Tim
    Commented Jan 3, 2022 at 17:09
  • Sure, but cast iron doesn't break as easily as epoxy. It also expands & contracts with heat/cold. I know it's marginal, but one would crack under stress whilst the other wouldn't.
    – Tetsujin
    Commented Jan 3, 2022 at 17:20
  • 2
    I would clean the inside of that iron pipe with a rotary wire brush to get the rust and scale off of it before attempting to glue anything to it. Commented Jan 3, 2022 at 18:03
  • 2
    @Tim - Pressure or not, you want the best chances for a water-tight seal.
    – gnicko
    Commented Jan 3, 2022 at 19:17

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