new downstairs toilet

best remove all of cast iron and refit plastic,if you do cut out a section you should not insert plastic section under cast iron as if at sometime the stack fixings fail the stack can collapse ,you can insert timesaver spun iron pipe and fittings.

Hi John, best I could advice is get someone who knows and has done this before. Make sure you measure and double check before cutting. You'll find out very quickly if the stack is loose or has no support or could fall. Prepare and know what to do if the stack is loose. So back to what I said before.... unless you know or are well prepared, get someone who knows cause you could loose the use of the upstairs toilet and bath/shower. An emergency repair costs a lot more.