Difference Between NPT & NPSM

If you have a National Pipe Thread (NPT) pipe and a National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM) pipe, it will be obvious that the two are different. A male NTP cannot fit into an NPSM pipe. Although the thread angles run at 60 degrees on both pipes, the NTP pipe tapers slightly whereas the NPSM pipe remains straight.

National Pipe Thread

National Pipe Thread (NPT) is an established proportion for screw threads on pipes. The threaded part of an NPT pipe tapers slightly, at a 1.7899-degree angle. The pipe's diameter decreases by 3/4 inches per foot of pipe length. National Pipe Thread pipes usually range from 1/8 inches to 2 inches in diameter. NPT pipes are not as useful as other kinds of pipe at wider diameters. The National Pipe Thread convention is sanctioned by the American National Standards Institute, a nongovernmental organization that establishes standards for American industry.

National Standard Free-Fitting Straight Mechanical Pipe Thread

The National Standard Free-Fitting Straight Mechanical Pipe Thread (NPSM) piping convention refers to pipe thread that is not tapered. NPSM pipe threads do not seal as effectively as NPT threads.

Other Names

Both NPT pipes and NPSM pipes go by other names. NPT pipes are also known as male iron pipe, female iron pipe, iron pipe thread, female pipe thread and male pipe thread. NPSM is sometimes also known as NPS.
Uses
Threaded pipes such as NPT seal tightly and are used to move liquids and gases. NPSM is not typically used to transport water or gases without an O-ring. Instead, they are used as structural pipes unless a sealer like an O-ring or tape is applied.