It is usually best to ream the hole with an NPT taper reamer before tapping. Depth of tapping is very critical, be sure to use something to gauge the thread as you go. Because the tap is cutting a taper, is has to cut all along its length. This can make the tapping operation very difficult. Use a very strong tap wrench when tapping an NPT thread.
Taps are available in various lengths.
The Extra Length or Pulley Taps can be handy when tapping in inaccessible places. Inside a pulley is a good example. Nut taps are a special long length tap that has a shank small in diameter than the inside of the thread. They are designed to be able to cut the threads in several nuts before having to stop and remove the nuts. The threaded nuts sit on the shank like a string of beads.