National Pipe Tapered (NPT)

Several different standards exist for tapered-thread pipe fittings. For each standard, the angle of

the thread is fixed, as is the angle of taper. Thread pitch (the number of threads per unit length) varies with the diameter of the pipe fitting.

For example, 1/8 inch NPT pipe fittings have a thread pitch of 27 threads per inch. 1/4 inch and 3/8 inch NPT fittings are 18 threads per inch, 1/2 inch and 3/4 inch NPT fittings are 14 threads per inch, and 1 inch through 2 inch NPT fittings are 11.5 threads per inch.

NPT threads have an angle of 60o and a taper of 1o 47’ (1.7833o):

NPT pipe threads must have some form of sealant applied prior to assembly to ensure pressuretight

sealing between the threads. Teflon tape and various liquid pipe “dope” compounds work well

for this purpose. Sealants are necessary with NPT threads for two reasons: to lubricate the male

and female pieces (to guard against galling the metal surfaces), and also to fill the spiral gap formed

between the root of the female thread and the crest of the male thread (and visa-versa).

NPTF (National Pipe Thread) pipe threads are engineered with the same thread angle and pitch

as NPT threads, but carefully machined to avoid the spiral leak path inherent to NPT threads.

This design – at least in theory – avoids the need to use sealant with NPTF threads to achieve

a pressure-tight seal between male and female pieces, which is why NPTF threads are commonly

referred to as dryseal. However, in practice it is still recommended that some form of sealant be

used (or at the very least some form of thread lubricant) in order to achieve reliable sealing.

ANPT (Aeronautical National Pipe Tapered) is identical to NPT, except with a greater level of

precision and quality for its intended use in aerospace and military applications.

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