Machinery's Handbook, 31st Edition
American Pipe Threads 2093 Table 4. Internal Threads in Pipe Couplings, NPSC for Pressure-Tight Joints with Lubricant or Sealer ANSI/ASME B1.20.1-2013 (R2018) Nom. Pipe Size Thds. per Inch Minor a Dia. Pitch Diameter b Nom. Pipe Size Thds. per Inch Minor a Dia. Pitch Diameter b Min. Min. Max. Min. Min. Max. 1 ∕ 8 27 0.340 0.3701 0.3771 1 1 ∕ 2 11 1 ∕ 2 1.745 1.8142 1.8305 1 ∕ 4 18 0.442 0.4864 0.4968 2 11 1 ∕ 2 2.219 2.2881 2.3044 3 ∕ 8 18 0.577 0.6218 0.6322 2 1 ∕ 2 8 2.650 2.7504 2.7739 1 ∕ 2 14 0.715 0.7717 0.7851 3 8 3.277 3.3768 3.4002 3 ∕ 4 14 0.925 0.9822 0.9956 3 1 ∕ 2 8 3.777 3.8771 3.9005 1 11 1 ∕ 2 1.161 1.2305 1.2468 4 8 4.275 4.3754 4.3988 1 1 ∕ 4 11 1 ∕ 2 1.506 1.5752 1.5915 … … … … … a As the ANSI Standard Pipe Thread form is maintained, the major and minor diameters of the internal thread vary with the pitch diameter. All dimensions are given in inches. b The actual pitch diameter of the straight tapped hole will be slightly smaller than the value given when gaged with a taper plug gage as called for in ANSI/ASME B1.20.1. Railing Joint Taper Pipe Threads, NPTR.— Railing joints require a rigid mechanical thread joint with external and internal taper threads. The external thread is basically the same as the ANSI Standard Taper Pipe Thread, except that sizes 1 ∕ 2 through 2 inches are shortened by 3 threads and sizes 2 1 ∕ 2 through 4 inches are shortened by 4 threads to permit the use of the larger end of the pipe thread. A recess in the fitting covers the last scratch or imperfect threads on the pipe. Straight Pipe Threads in Pipe Couplings, NPSC.— Threads in pipe couplings made in ac- cordance with the ANSI/ASME B1.20.1 specifications are straight (parallel) threads of the same thread form as the ANSI Standard Taper Pipe Thread. They are used to form pressuretight joints when assembled with an ANSI Standard external taper pipe thread and made up with lubricant or sealant. These joints are recommended for comparatively low pressures only. Straight Pipe Threads for Mechanical Joints, NPSM, NPSL, and NPSH.— While external and internal taper pipe threads are recommended for pipe joints in practically every ser - vice, there are mechanical joints where straight pipe threads are used to advantage. Three types covered by ANSI/ASME B1.20.1 are: Loose-fitting Mechanical Joints With Locknuts (External and Internal), NPSL: This thread is designed to produce a pipe thread having the largest diameter that it is possible to cut on standard pipe. The dimensions of these threads are given in Table 5. It will be noted that the maximum major diameter of the external thread is slightly greater than the nominal outside diameter of the pipe. The normal manufacturer’s variation in pipe diameter pro vides for this increase. Loose-fitting Mechanical Joints for Hose Couplings (External and Internal), NPSH: Hose coupling joints are ordinarily made with straight internal and external loose-fitting threads. There are several standards of hose threads having various diameters and pitches. One of these is based on the ANSI Standard pipe thread and by the use of this thread series, it is possible to join small hose couplings in sizes 1 ∕ 2 to 4 inches, inclusive, to ends of standard pipe having ANSI Standard External Pipe Threads, using a gasket to seal the joints. For the hose coupling thread dimensions see ANSI Standard Hose Coupling Screw Threads starting on page 2101. Free-fitting Mechanical Joints for Fixtures (External and Internal), NPSM: Standard iron, steel, and brass pipe are often used for special applications where there are no internal pressures. Where straight thread joints are required for mechanical assemblies, straight pipe threads are often found more suitable or convenient. Dimensions of these threads are given in Table 5.
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