I’ve got an older house with 4″ Cast Iron DWV. A basement apartment has a 2″ Steel DWV screwed directly into 90 degree 4″ Cast Iron elbow at bottom of toilet. I need to replace the 2″ Steel DWV with PVC, but from prior experience fear that the 2″ Steel DWV will simply break off at the threads (leaving the threads imbedded in the Cast Iron elbow, which will be a bear to try to fix). My guess to try to mitigate the problem would be to first hacksaw the steel pipe off about .5 to 1 in from the cast connection (instead of trying to unscrew the pipe, which will probably cause the threads to simply snap off). Then, using a hacksaw, make one or two cuts directly into the remaining steel pipe, at 90 degrees towards the cast fitting. Then, carefully make some cuts on the inside of the steel pipe at the location of the 90 degree cuts, being careful not to cut into the threads. Then, take a punch and try to knock the steel pipe threads out of the cast iron pipe. Assuming this is successful, then I can simply screw in a 2″ PVC DWV fitting and complete the job. Is there a better way to do this, or should I consider converting the 4″ cast iron over to PVC as well? (The 4″ cast iron works fine).