An FIP is a "female iron pipe," or one in which the threads are on the inside.
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Simply, FIP is pipe that is threaded on the inside. As a female, it accepts the male pipe, which is threaded on the outside. The male end is twisted inside the female until it is snug. In order to keep a proper seal, Teflon tape is needed around the male threads.
Threaded pipes provide an effective seal for pipes transporting liquids, gases, steam, and hydraulic fluid. They are used for everything from shut-off valves to water heaters.
Aside from iron pipes, female threads are used in copper, steel, brass, PVC, nylon and bronze pipes. But not all of the pipes are interchangeable. Before using a particular type, check to see if it will be compatible with what will pass through it.
The most common FIP sizes are 1/8, 1/4, 1/2, 3/4, 1, 1 1/4, and 2 inch, with smaller sizes used mainly for compressed air. You rarely will find larger sizes, and hardly ever in a residence.
Threading Your Own
Unthreaded pipes can be threaded with a pipe threader. Use cutting oil and place the die inside the pipe for a female thread, then turn. Be careful, though, as pipe threads are usually sharp.