ok il try again ,hire a snap cutter or wheel cutter to cut the cast ,can be done with disc cutter or even hacksaw but really it isn't worth the hassle.
Check where your going to cut cast iron is way heavier than it looks ,check the bolts are holding pretty well before you start as when you cut it you're going to remove its self support.
If there at all dodgy or you doubt them ,its probably better to remove the whole stack from bottom and do it all in plastic or get a professional in.
Find a suitable section to cut out try to not remove any fixing points unless you really have to.
Cut out section get 2x rubber drain couplings and slide then up and out the way ,cut pipe section to fit gap or whatever you doing there ,drop couplings do em up and away you go.and away you go
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