Connecting plastic to cast iron drain line

[adrianmariano]

I'm replacing a cracked utility sink. The drain line for this sink is either copper or steel connected into cast iron. (There is a big T-fitting on the cast iron.)

In order to make a connection for the new sink I would like to connect a plastic pipe to the cast iron. Now I could do it using a fernco---just leave some of the existing copper/steel intact---but this does not seem like the best solution. In three other places where this situation has arisen in my house, a direct connection between plastic (ABS in two cases, PVC in one case) has been made at the cast iron fitting.

But when I search around for information on how to make such a joint I come up empty and I see lots of advise to use clamps. How can I connect plastic piping directly into the cast iron? Is there some reason why ABS would be preferred for this? (In both cases where ABS was used it is a short section that makes a transition to PVC, like PVC is preferred but there's some reason not to use PVC at the cast iron-plastic junction.)

[Gary Swart]

If the cast iron tee is threaded rather than leaded, you can get a PVC or ABS adapter that will just screw into the tee. If it is a leaded joint, then the easiest way would be to use a banded coupling and leave the connection alone. A banded coupling is not a Fernco. It has a stainless steel band the entire length of the fitting. A Fernco is not approved for inside work that will not be buried. IMHO, although PVC and ABS are different materials, there is not practical difference.

[adrianmariano]

I thought the joints looked threaded. Is there any way to tell for sure? If it's threaded I should be able to simply unscrew the old pipe? And when I go to install the ABS/PVC in there, what material do I use to seal the joint at the threads? (The joints I see look kind of like they have plumber's putty on them.)

[jadnashua]

Plumber's putty will work fine. If you can't see a band of lead, then it is threaded. Normally, you can still see some of the threads, since it is a taper (interference) fit, and rarely is threaded all the way in.

[markts30]

The "plumbers putty" is probably dried pipe dope...
I would use teflon tape on the male plastic threads going into the cast iron and pipe dope on the cast iron female threads...

As well, thread the plastic into the cast by hand then snug it up using a pair of channel locks - do NOT make it "really tight" as you will probably either break the plastic or strip the threads.

Do NOT cross thread the plastic into the cast iron - it is easy to do and will leak like a seive.

[jadnashua]

I meant to say pipe dope, not plumber's putty. Teflon tape works, too.

[Randyj]

TFE plumbing paste/pipe compound works best when mixing metal and PVC...in my opinion.

[adrianmariano]

Thanks to everybody for the help. I've got some Oakley Great White Pipe Joint Compound with Teflon so I'll just use that.

[BAPlumber]

The "plumbers putty" is probably dried pipe dope...
I would use teflon tape on the male plastic threads going into the cast iron and pipe dope on the cast iron female threads...

As well, thread the plastic into the cast by hand then snug it up using a pair of channel locks - do NOT make it "really tight" as you will probably either break the plastic or strip the threads.

Do NOT cross thread the plastic into the cast iron - it is easy to do and will leak like a seive.

years ago a boss of mine said if you used ABS cement as a thread sealant when connecting abs to Cast fittings it made the best connection. I've tried it a few times and it seems to work.