I replaced a lavatory in the bathroom with a new faucet and new trap, but the old 1-1/4 trap is connected to a cast iron pipe with led through the wall. I cut the old trap out, and I used a 1-1/4 connector and it is okay. I'd like to replace the old piece of trap that is leading into the cast iron pipe with a straight PVC pipe and an adapter. What is the best way to do that?
You would normally cut out the piece right before - 8 inches or so before the main stack. You then connect PVC to cast iron with a coupling.