I've coupled PVC to old (unaccessible) lead drain pipe and to cracked/cut
cast iron drains, but now I'm on a new project in my new house. I have cast
iron in the basement, and along the way there are a few y's with the hex-
nut screw cap on them.

I'd like to remove one of those caps and use PVC to tie in a new 2" drain
for my kitchen- the old drain is (at best) 2" iron and barely drains at
all; and it is inaccessible, so I'd just cap it under the kitchen sink and
run the new PVC in from another direction and up in the original location.

However, before I take the cap off the cast iron, I'd better ask...I'm
guessing that a standard threaded PVC coupling isn't going to fit the
threads on the cast iron (at least not with any type of effective seal).
What is the preferred method of making this type of junction? I can use a
rubber coupling again, although I don't have enough length of iron pipe to
cut it, so it would be over the large end where the cap sits, and I'm not
sure if a rubber coupling would have enough room to seat well.

Thanks for your suggestions,
Keith