Begin by ensuring no water will flow through the drain pipe during the repair. Inform everyone in the house not to use any sinks, showers, or flush any toilets. Clear the workspace directly below the damaged pipe section. Lay down heavy-duty plastic sheeting to catch dust, debris, and any residual water. Gather all your tools and materials and place them within easy reach. Most importantly, put on your safety equipment: safety glasses, heavy-duty work gloves, and a respirator to protect against cast iron dust.

Cast iron is brittle and incredibly heavy. Before cutting, you must prevent the pipe from sagging or falling. On each side of the section you intend to remove, install robust supports. You can use strong metal strapping (pipe hangers) secured to the ceiling joists above, or build temporary supports from 2x4 lumber wedged firmly between the floor and the pipe. Riser clamps that bolt around the pipe and can be fastened to a support structure are an excellent option. The supports should be as close to the cut lines as possible without interfering with your saw.

Identify the cracked area. Using a marker, mark two cut lines on the pipe, one on each side of the crack. Ensure the marks are at least 2-3 inches away from the visible damage and any nearby pipe joints or fittings. Install a new blade designed for cutting metal or cast iron (diamond grit or carbide grit is best) into your reciprocating saw. Apply some cutting oil to the blade or the cut line to reduce heat. Begin cutting at your first mark, using firm pressure but letting the saw do the work. The process will be slow, loud, and will create sparks. Once the first cut is complete, repeat the process on the second mark. Have a helper ready to assist in controlling and lowering the heavy, cut section of pipe into a bucket or onto the plastic sheeting.

For the new flexible couplings to create a perfect seal, the ends of the remaining cast iron pipe must be clean and smooth. Use a heavy-duty wire brush to scrub the outside surface of both pipe ends, removing all loose rust, scale, and grime. If there are any significant burrs or high spots left from the cutting process, use a metal file to smooth them down. The goal is a clean, uniform surface approximately 2-3 inches from the end of each pipe, where the rubber gasket of the coupling will sit.

First, test-fit the shielded flexible couplings by sliding them onto each of the prepared cast iron ends. Don't tighten them yet. Now, measure the exact distance between the internal pipe stops inside the two opposing couplings. This measurement is the precise length your new PVC pipe needs to be. Transfer this measurement to your new piece of Schedule 40 PVC pipe and mark it. Use a PVC pipe cutter, handsaw, or miter saw to make a clean, square cut. After cutting, use a deburring tool or a simple utility knife to shave off any plastic burrs from the inside and outside edges of the cut.

Slide one of the flexible couplings completely onto one of the cast iron pipe ends. On the other cast iron pipe, slide the second coupling on but then roll the rubber gasket back over itself, exposing the inside of the coupling. Now, insert one end of your new PVC pipe fully into the first coupling until it hits the internal stop. Carefully flex the PVC pipe into position and guide the other end into the second coupling. Once it's in place, unroll the rubber gasket so it sits flat against the PVC pipe. Adjust the PVC pipe and both couplings so that everything is aligned and the pipe is seated against the stops in both couplings. The metal shields should be centered over the rubber gaskets.

Using a nut driver or, preferably, a torque wrench set to the manufacturer's specification (usually 60 inch-pounds, often printed on the stainless steel band), tighten the clamps on both couplings. Tighten the screws on each band evenly, alternating between them if there are multiple, to ensure even pressure. The band should cinch down firmly. Once both couplings are fully tightened, it's time to test. Go to an upstairs fixture and run a significant amount of water (e.g., flush a toilet a few times, run a sink for a minute). While the water is running, use a flashlight to carefully inspect the top and bottom of both couplings for any sign of dripping or leaking.