If you have an older home, you probably have metal pipes running somewhere through your house. Connecting metal pipes to newer plastic ones can be a headache, but with a few simple steps, you can get the job done.
Make sure your metal pipe is a female-threaded pipe the same diameter as your plastic pipe. A female-threaded pipe is the pipe you screw the other pipe into, and its threads are inside the end of the pipe.
To get the connection started, screw the end of your male-threaded plastic pipe into your female pipe just past the first thread.
Wrap Teflon tape tautly over the threads of the plastic male-threaded pipe (the male-threaded pipe has its threads on the outside of the end of the pipe) in the same direction as the female threads, but be careful not to wrap the tape too thick. Three or four times around should be sufficient to create a good seal.
Finish screwing the plastic pipe into the metal pipe. If your Teflon tape is thick enough to seal, screwing the pipes together should be a little bit difficult. Make sure the pipes are tightly screwed together.
Turn on your water, and check for leaks.
If you have a leak, your Teflon tape may not be thick enough. Unscrew the pipe, take off the Teflon tape, and repeat steps 2 through 5.
Plumbing sealants other than Teflon tape may be used. However, Teflon tape usually creates the least mess and is not caustic to plastic. Teflon tape comes in a variety of widths but is most common in 3/4-inch to 1-inch widths.
Do not try to attach a male-threaded metal pipe to a female-threaded plastic pipe. Since metal is harder and denser than plastic, the plastic will eventually leak. If your leak is in a male-threaded metal pipe, call a professional plumber as part of the metal may need to be cut off to accommodate the switch to the plastic pipe.