had to put in a new basement sink, the other one had a crack in it. it had all the old metal piping for the drains. i changed everything over to pvc except the cast iron piece in the floor. the piece in the floor is about 5 1/4in in diameter from outside to outside. the 4 in rubber sleeve is to small and the 6 in rubber sleeve is to big. what can i use to run the 1 1/2 pvc into the 5 1/4 in cast iron.
You just need to go to a plumbing supply house where they know what you need, which will involve a no-hub coupling made for cast iron-to-PVC. It sounds like you're trying to use a standard Fernco coupling.