If you have cast iron drain (waste) pipes they are likely joined with a lead & Oakum seal. Oakum is a hemp material coated in tar, it is placed into the joint to seal it (Oakum expands when it gets wet to seal the joint). The Oakum is held in place (and the pipe joint is given some strength) by casting a bead of lead about 1″ thick around the pipe inside the hub/socket. In this picture the cast iron pipe hub is on the left.
It is possible to remove the lead and oakum. After you have gotten all the lead out and cleaned up the hub, you can place a donut (also known as a compression joint seal) in the hub of the cast iron pipe to adapt it to PVC. (Note that donuts are only usable on non-pressurized pipes, such as waste/drain lines.)
I have manually cleaned out a lead & oakum cast iron joint. It sucks. My recommendation is to cut the pipe somewhere other than at the lead & oakum joint and then use a Flexible Coupling to join it to your PVC. Yes, this introduces discontinuities in the waste water flow, and gunk may collect and build up at these edges, possibly leading to plugged pipes in the future….but that is small price to pay for avoiding having to manually unseal a lead & Oakum joint. For the gory details of how to manually disconnect and clean out a lead & Oakum joint, read on…
If you were a professional, and the lead & oakum joint was well away from any wooden structure, you would have the correct respirator and high output torches that would allow you to simply melt the lead in one go. But this is practically impossible inside walls or for the home handyman without renting some fancy equipment.
If you are reading this trying to figure out how to remove the joint, you don’t have access the the proper respirator, high output torches, or the joint is too close to a joist, stud, or other wooden structure. It is possible to physically remove the lead by drilling into it every 1/4″ or so to separate it out, and then chipping out each separate piece with a 1/2″ or 1/4″ chisel. This takes a long time and a lot of patience and effort. If you do not have easy access to the joint (for example, if it is in between two joists or studs) you will have to use a right-angle drill. Use a reciprocating saw (sawzall) to trim the incoming spigot/pipe that you are replacing about 1/2″ away from the hub so that you can easily drill around it.
Re-evaluate if you REALLY want to keep this joint in place. Why not tear out another panel of drywall and just cut the pipe and use a flexible coupling to join up to it? Really, replacing drywall is relatively easy compared to slowly chipping out the lead from one of these joints with a drill bit. Of course, to break cast iron pipe cleanly requires a specialized pipe breaker tool that you would probably have to rent.
If you decide that due to space or other constraints (such as a concrete pad) you NEED to save and re-use the joint….find the cheapest drill bits you can. Don’t use your fancy titanium carbide drill bits for this job. You want steel drill bits that are harder than the lead, but otherwise as soft and non-brittle as possible. Otherwise, when you are drilling and put any amount of sideways pressure on them, the drill bits will shatter because they are super high hardness (and very brittle). This leaves a solid chunk of very hard to remove super high hardness metal embedded in your lead seal. Don’t use a drill bit as a reamer! After you finally get the lead chipped away enough to remove the inner pipe, the inside of your hub will still have a thin ring of lead all the way around. Remove all of the Oakum using pliers, scrapers or a rotary tool. (Because Oakum will burst into flame if you get it hot and expose it to air, it’s covered in tar after all…)
At this point you can use a small blowtorch to slowly melt the remaining lead. Use the torch INSIDE the pipe, and have a fire extinguisher and a bucket of water standing by! Drops of lead will start to fall from the ring. Eventually the cast iron pipe itself will get hot enough that it will melt the lead touching the pipe. Now you can use a screwdriver or chisel to pry the entire ring of lead out of the pipe. (You’ll probably have to hit the inside of the hub with a good scrubbing from a Brillo pad or sandpaper, AFTER IT COOLS OFF.)
Now you have a clean hub connection to fit your donut into. (Don’t you wish you had cheated with a flexible connector now?) Test the fit and make sure everything will line up first.
Then use some cooking oil spray inside the cast iron pipe hub to make it easier to hammer the donut in. Lay a bead of silicon caulk around the donut (it will spread out and need to be cleaned up around the outside once you are done hammering the donut in.) Hammer the donut in using a large hammer (or side of a small hammer if in an enclosed space between studs.)
Now you can chamfer the end of the PVC pipe to make it easier to go into the seal/donut. Mark your PVC pipe with inch markings so you can visually tell how far it has gone into the seal. Spray some more cooking oil inside the donut, place a bead of silicon caulk around your PVC pipe, and hammer it in.