If you get the right size, you can insert the pipe in it with some effort, but it's not horrible. If you get one that is too tight, you'll be swearing, if you get it in at all. If it's too loose, it will leak. Get the right one and it works great. I was in the same situation...needed to get it done and the local shop only had a couple of sizes, neither one fit...bummer. I ended up cutting it off and replacing with pvc.
There is an accepted method of packing and caulking the joint that would allow you to use PVC (or you could pay a plumber to do a leaded joint with CI). I'd bought a tube of the special caulk and got some of the oiled hemp, but the stuff was (unopened, but) so old, I couldn't use it...was really pissed at the supply house - they should have known (when I looked at the date code, it was like 6-years old). Packing the stuff in, is a pain, but the size doesn't matter if yo ucan find stuff beyond the expiration date. Can't remember the name of the stuff. You have to pack a layer of hemp, caulk, and repeat, three times, filling up the socket.