Not all cast-iron fittings have bell-and-spigot inlets. Some older types accept threaded pipes or threaded adapters. You'll find these fittings in homes that have cast-iron stacks with copper or galvanized-steel branch liens. These threaded fittings offer yet another method of adapting plastic to cast iron, though not always as easily as with banded coupling. They also have the advantages of being extremely inexpensive and making neat, professional-looking retrofit connections.

    In the case of copper or brass threaded adapters, just cut the old copper line near the cast-iron fitting, and back the adapter out with a large pipe wrench.

    Galvanized-steel threads, in contrast, fuse with the cast iron over time. To break these threads free , you often need to heat the cast iron with a torch. Heat causes the female half of the fitting to expand slightly, loosening its grip. In any case, when you've removed the old threaded piece, clean the rust from the cast-iron threads using a wire brush. Then wrap plumber's pipe threaded-sealing tape-only two full rounds-onto the threads of the new plastic adapter, and screw the adapter into the cast-iron fitting.