NPT is a letter designation for American Standard Pipe Taper Thread. NPT pipe threads have a taper cut into the male and female fittings. The taper placed on both sets of threads allows for a solid mechanical joint that does not leak liquid or gas. In many assemblies, a sealant is placed over the male pipe threads to aid in sealing the joint. The sealant will fill voids between the male threads and female threads as the fitting is tightened. Tightening a NPT fitting is a two-part process that requires a pipe wrench.
Insert the male thread into the female fitting.
Turn the male pipe thread in a clockwise direction. Do not cross the threads at this stage. If the fitting appears to bind or stop moving after a full revolution, unscrew the fitting. Look inside as there may be a foreign substance on the threads. Clean the threads. Reinsert the male fitting into the female.
Continue to turn the male pipe fitting in a clockwise direction until you can no longer do so, approximately two to four full revolutions depending on the size and type of fitting. Larger fittings will turn more easily than smaller-diameter fittings.
Fit the pipe wrench onto the outside of the male pipe fitting. Close the jaws using the thumbscrew adjustment. Moving the thumbscrew in a clockwise direction closes the jaws, while a counterclockwise direction opens the jaws.
Turn the pipe wrench two full revolutions in a clockwise direction. You should have a tight and sealed joint between the male and female fitting.