Main drain stack reconfiguration

I am remodeling my 50's rancher. The remodel involves reconfiguring upstairs toilets and baths, as well as gutting and finishing the basement. The basement ceilings are a mere 88 inches. The low ceilings are compounded by the drain design - a 4.5 inch cast stack is mortared into an interior, load-bearing cinderblock wall, and the two toilet drains (one PVC and one cast) are tapped into the stack such that they run under the floor joists. The same is true with the secondary drains and vents (some cast, some PVC). The result is a complete, chopped-up mess of soffits and dropped ceilings.

My plan is to replace the cinderblock wall with a framed double-wall, housing a cast L-beam with support columns. I'll then reconfigure the drain stack in this framed double-wall, so that all drains can run between joists and then drop into the stack behind the double-wall. I'll install a 24" standard interior door for complete, top-to-bottom access to the main drain stack. This seems fairly straightforward.

My question is how to handle the transition from cast to PVC. Even if I use cast for the toilets, I'll have to tie in PVC somewhere for the secondary drains. Won't it be easier to simply select a point at which to transition the main stack to PVC, and then run all drains in PVC? If so, my plan would be to cut the stack at about the 32" mark, just below the current access plug (not sure if this is the correct term, but its the nipple that allows access for, I assume, clearing out the main drain), and put the new access plug into the PVC section (that way I can re-orient it at the same time so that it faces the planned door frame, since it is currently oriented in a way that prevents practical access).

Is this all clear as mud?

It's even better than mud.
It sounds good.
What size is the old cast on your outside dimension?

It looks like 4.5 inches, but that's an estimate by holding a tape measure across the pipe.

It sounds then like I have a good plan, but I need help understanding how to make the cast-to-pvc conversion. If I cut the stack at the 32" mark, my questions are:

1 - are there any precautions I need to be aware of, so that I don't mess something up?
2 - how do I make the cut (ie: what tool, and how critical is it to make a 100% straight, true and level cut)?
3 - what parts/procedures/materials are required for transitioning/joining the now-severed main to PVC?

Thanks for the help!

Cast iron is very heavy , so if there is a possiblity it could move after it is cut, you must support it well prior to the cut.

The quickest way to cut cast iron is with a soil pipe or snap cutter (same thing). This is a heavy duty chain device that you wrap around the pipe, then using the ratchet wrench tighten it up until the pipe snaps. It has hardened pointed rollers in the chain that cause the pipe to fracture along their point around the circumference. Takes all of about a minute or two. You can rent one at a tool rental place - I got one at a HD last time. Cutting with a saw takes forever and you'll likely spend as much on blades as the rental of the snap cutter, even assuming you have a saw that can do it.

Depending on whether it is underground or in the house will dictate the connector needed to join the two pipes: underground, you use what looks like a big rubber sleeve with hose clamps at either end. You must backfill the pipes well to support them and keep them from shifting, as the sleeve has little ability to keep them in alignment. If it is above ground, you need a no-hub connector which is basically a neoprene rubber sleeve with a metal reinforment sleeve over it. It also has hose clamps at either end. The metal sleeve keeps the two ends aligned above ground when you can't backfill around it.

Don't understand the coupling

Thanks for the tip on cutting.

This is a vertical pipe, coming up from the slab and into the house. The clamped rubber sleeve doesn't sound like it will support the stack structure above the sleeve. It also seems untuitively like a future point of failure. Am I missing something?

There is a place in the cast stack where they've already tapped in a PVC run for one of the toilets. The stack has a cast sanitary tee, and into that they've inserted a PVC 4" collar/coupler. There is some kind of glue or epoxy sealing it (hard, but you can gouge it with a fingernail). The PVC run then goes on from there. See attached .JPG.

I guess this was the kind of solution I anticipated for the coupling. Not doubting, just verifying...


ps - note that the image may be disorienting. The stack is running up through the wall, not the floor. The PVC coupling is coming out of the wall. In other words, the arrow labeled "stack flow to slab" is pointing in the direction of the floor (slab).

If you have a hub, you can get a donut fitting that will make a proper seal between the pvc and the hub...epoxy is not proper, and it looks like that one leaks! There is an approved pseudo leaded caulk that can be used in a hub, but it is a pain - it requires three layers of a packed hemp/caulk pair. The no-hub connectors are designed to make the proper connection, and if the pipes aren't corroded and the connector is properly torqued, will be fine and never leak. You DO want to ensure that the cast iron sections above are properly supported. They may NOT be now. The proper clamps are bolted to the CI and either lay across supporting beams (say in the attic across joists), or are bolted to a structure strong enough to hold it. Then, the only thing the no-hub needs to support is lateral deflection, and provide the waterproof seal...it is designed specifically for that.