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

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.

Replace enough iron with plastic until all you have left is simple pipe-to-
pipe connections, which will connect with the rubber couplings. The iron is
likely old and going to make more trouble, so the less of it the better.

I would screw in a cast iron male-to-female adapter(with pipe dope)
after removing the cap, then put the pvc in this with a lot of the gray
stuff- think it's called 'plastic weld' making sure pvc is clamped
well enough that it will not move.

Reason I suggested it was he said he wasn't sure he had enough room on
the Y to attach rubber coupling.