The National Pipe Straight Mechanical thread chart provides the thread allowance, major diameter and pitch diameter of the external thread, as well as the minor diameter and pitch diameter of the internal thread for each of the nominal pipe size. It also lists the threads per inch.
The NPSM thread chart provides the American Pipe Threads Standard for the free fitting mechanical joints for the fixtures. The chart provides the maximum and minimum value of the major diameter and the pitch diameter of the external thread. It also provides the maximum and minimum value of the minor diameter and the pitch diameter of the internal thread.
The chart shows that the diameter of the pipe determines the major diameter of the external thread. The major and minor diameters of the internal thread and the minor diameter of the external thread vary according to the pitch diameter.
The listed minor diameters of external threads and major diameters of internal thread apply to the commercial straight pipe dies and the commercial ground straight pipe taps. The diameters result from the addition of the truncated thread depth and the maximum pitch diameters. The thread size is the nominal bore of the pipe. The units of the measurements within the chart are in inches.