Pipe Threads, General Purpose (Inch)
This Standard covers dimensions and gaging of pipe
threads of the following series:
1.2 Related Standard
Hose coupling joints are ordinarily made with straight
internal and external loose-fitting threads. There are sev-
eral standards of hose threads having various diameters
and pitches, one of which is based on the American
National Standard Pipe Thread. By the use of this thread
series, NPSH, it is possible to join small hose couplings
in sizes 1⁄2 to 4, inclusive, to ends of standard pipe having
American National Standard external pipe threads,
using a gasket to seal the joint. For dimensions, toler-
ances, and gaging, see ASME B1.20.7.
1.3 Thread Designations
1.3.1 The types of pipe threads included in this
Standard are designated by specifying in sequence the
nominal pipe size, number of threads per inch, and the
thread series symbol as follows:
Decimal equivalent notation may be substituted for
fractional pipe sizes. For example
For left-hand threads, add “LH” to the designation.
Designations without “LH” will signify right-hand
1.3.2 Each of these letters in the symbols has signif-
icance as follows:
N p National (American) Standard
P p Pipe
T p Taper
S p Straight
C p Coupling
R p Railing Fittings
M p Mechanical
L p Locknut
1.4 Sealing (NPT and NPSC Only)
1.4.1 Mating Threads. Mating threads should
always contact on the thread flanks. The design toler-
ances are such that mating crests and roots may clear,
contact, or interfere (see Fig. 1). This joint may not neces-
sarily seal, unless a sealant is used.
1.4.2 Sealant. Where pressure-tight, leak-free joints
are required, it is intended that threads conforming to
this Standard be made up wrench-tight with a sealant.
To prevent galling during installation, the sealant may
have lubricating properties.
1.4.3 Tightening Torque. Due to application-specific
variables such as materials, wall thickness, operating
pressures, etc., no guidance is given in this Standard
regarding joint-tightening torque. However, joints
should be tightened beyond the hand-tight engagement
position. Advancing the joint past hand-tight creates
interference between external and internal thread flanks,
produces a seal (with the use of a sealant), and helps
prevent loosening of the joint. Overtightening may be
detrimental to the sealing function of the joint.
1.4.4 Other Considerations. Out-of-roundness of
mating parts can negatively affect their ability to seal
when made up wrench tight. The product’s elasticity
and ductility will also affect sealing.
1.4.5 Pressure-Tight Threads Without Sealant. Pipe
threads designed for pressure-tight joints that may be
used without sealing compounds (Dryseal Threads) are
covered in ASME B1.20.3.
Useful and supplementary information that is not a
part of this Standard is presented in a nonmandatory
appendix. Specifically, the nonmandatory appendices
cover the turns of engagement method of gaging, sug-
gested prethreading hole diameters, and an explanatory
Copyright c 2013 by the American Society of Mechanical Engineers.
No reproduction may be made of this material without written consent of ASME.