Hi all, to be brief here's what I was trying to do. My drainage stack is all cast iron and has a Y at the floor for the clean out plug. The problem was I had no drainage for a washer/laundry sink, so the solution was to cut out the cast iron where needed and replace it with PVC, installing an extra Y low enough to tie in the washer/laundry sink drainage. Now that I have cut the pipe in two places, the weight of it has closed the gaps (although not sealed them which means it now leaks). My question to you is how do I safely go about removing this section of "loose" cast iron to put in the PVC? Some fittings to the kitchen and bathroom have been replaced with PVC already, but the vent and a few other things are still cast iron making it very heavy.

Cast iron always has to be supported before cutting. Did you do that? . You can put a pipe support clamp on the upper section and then use a bottle jack to lift it back up. Use Fernco sleeves on either side of the PVC / cast iron fitting.