I would like add a Y to an existing 4" cast soil stack. After making the cut there will be some play when the Y is inserted, I have a worm clamp, but how can I take up the slack so the pipe does not slide down. Also is a 3" cutting tool with the right blade good enough?

I hope you'll be installing a No-Hub Cast Iron Wye (sold only at a plumbing supply store) in the cast iron stack ...not a good idea to install a plastic (ABS or PVC) fitting in the middle of a cast iron stack.

Here, you purchase a RISER CLAMP (see images) or two, tighten it down about a foot or so above your top cut into the pipe and then block the riser clamp to the floor or nail it to the studs and install another, if needed at the floor (see images).

I suggest using a reciprocating saw with a DIAMOND sawzall blade (made exclusively by Lennox) or use a carbide tipped sawzall blade...both work pretty quick all in all!

Measure the length of the cast iron wye and add 3/4" to the measure and transfer that new measurement onto the pipe. Make your cuts and remove the piece. Install 2 SHIELDED CLAMPS to the pipe (slide up and down onto the pipe and then tighten temporarily to the pipe), install 2 rubbers onto the wye fitting being sure to "flip" the rubbers half way back on themselves and then insert into the pipe. Flip the rubbers up, position the fitting and then slide the clamps over the rubbers...tighten appropriately and then remove the clamp.

Did all that make sense??

Mark

Need more info such as total length go the stack and where you plan to cut. What are you adding?