NPSM vs NPSL: Same Pipe Size, Different Fit, Different Jobs

I have turned both NPSM and NPSL threads, and I keep a gauge for each.

What does NPS mean, and how does it relate to NPT?

In ASME B1.20.1, N stands for National (American) Standard, P for Pipe, T for Taper and S for Straight. On a straight thread the last letter says what it is for: C for coupling, M for mechanical, L for locknut.

All of them share the 60° thread angle and the same threads per inch for each pipe size: 27 for 1/8, 18 for 1/4 and 3/8, 14 for 1/2 and 3/4, 11.5 for 1. The standard sets the basic pitch diameter of NPSM and NPSL at E1, the pitch diameter of the NPT thread at its hand-tight plane. What changes is the taper: NPT tapers 1 in 16 on the diameter and seals on the threads with a sealant; the NPS threads are straight. How NPT differs from the 55° PT and PF threads is in the first article in this series.

NPSM is for mechanical joints that do not hold pressure

ASME B1.20.1 places NPSM in special applications where there is no internal pressure, and gives its dimensions for joints "where reasonably close fit of the mating parts is desired". Its limits follow the Unified system, Class 2A on the male and Class 2B on the female, and the minimum pitch diameter of the female thread is E1 itself.

In American catalogs the thread also turns up doing a different job. Dixon lists a 90° elbow with a female NPSM swivel nut and states that the swivel seals on a 30° seat: the straight thread only holds the nut on, and the pressure seal is left to the seat. Hydraulic swivel fittings are not covered here.

NPSL is made loose on purpose, for a locknut

ASME B1.20.1 sets the NPSL male thread at the largest diameter that is practical to cut on standard pipe, usually paired with a straight female thread for a loose fit. The maximum major diameter of the male is a little larger than the pipe itself: for 1/4, 0.541 in (13.74 mm) against a pipe outside diameter of 0.540 in (13.72 mm). Normal variation in pipe diameter takes up the difference. The example the standard gives is a standpipe fixed to the floor or wall of a water tank with a locknut.

McMaster-Carr files its pipe locknuts and conduit locknuts under thread type NPSL. Table 7 of the standard puts both NPSL pitch diameters well above E1 and leaves about one turn of clearance between the largest male and the smallest female.

What happens when NPSM and NPSL get mixed up?

An NPSM male at its largest, 0.4903 in, inside an NPSL female at its smallest, 0.5073 in, leaves 0.017 in (0.43 mm) on the pitch diameter. The other way round, the smallest NPSL male, 0.4986 in, is already 0.0012 in (0.03 mm) larger than the largest NPSM female, 0.4974 in. The same holds for 1/8, 3/8, 1/2, 3/4 and 1, with the smallest NPSL male sitting 0.0022 to 0.0061 in (0.06 to 0.15 mm) above the largest NPSM female.

I have mixed the two up. The parts went together, but the fit was loose, which is what Table 2 predicts for an NPSM male in an NPSL female.

A GO and NOT GO set checks a thread against one designation only; an NPSM set does not check an NPSL thread. A ring set checks a male thread, the GO ring for not too large and the NOT GO ring for not too small. A plug set, like the one in Photo 1, checks a female thread the other way round, the GO plug for a hole not too small and the NOT GO plug for a hole not too large.

An NPT L1 gauge is a different measurement altogether: a taper gauge, turned on by hand, read by where the end of the thread stands against the face or notch of the gauge. It does not stand in for a straight GO and NOT GO pair, which checks the cylindrical limits over the full length of the thread.

NPSC is a straight female thread made to take an NPT male

NPSC is the third straight thread in the standard, and the only one it describes as pressure-tight: a straight internal thread, used in pipe couplings, that forms a pressure-tight joint with an NPT external thread when made up wrench-tight with a lubricant or sealant. Its pitch diameter limits sit 1.5 turns large and 1.5 turns small of the NPT thread, so for 1/4-18 they run from 0.4864 to 0.4968 in (12.355 to 12.619 mm), either side of E1 at 0.4916 in (12.487 mm).

An NPSC female and an NPSM female overlap in pitch diameter, 0.4864 to 0.4968 in against 0.4916 to 0.4974 in. They are still different threads with different jobs: one takes a taper male and seals, the other takes a straight male and holds no pressure. Their limits are built differently, NPSC from the NPT pitch diameter and NPSM from Unified Class 2A and 2B, and each is accepted by its own gauge, an NPT taper plug gauge for NPSC and a straight GO and NOT GO plug pair for NPSM.

What should the drawing and the RFQ say?

Write the full designation for each thread, pipe size, threads per inch and series, the way ASME B1.20.1 writes it:

• 1/4-18 NPSM, for a straight mechanical thread with a close fit
• 1/4-18 NPSL, for a locknut thread with a loose fit
• 1/4-18 NPSC, for a straight female that takes an NPT male

Not "1/4 NPS", not "1/4 straight pipe", not "1/4 pipe".

If the part is plated, say whether the thread limits apply before or after plating.

Ask the supplier which gauge will accept each thread: a straight GO and NOT GO set for NPSM, a separate set for NPSL, and an NPT taper plug gauge for NPSC.

NPTF, the Dryseal taper pipe thread of ASME B1.20.3, has its own gauges and its own rules, and is not covered here.