PT, PF and NPT: Reading Old JIS Pipe Thread Callouts

Shops in Taiwan still use the old names. The thread gauges I work with are still engraved PT, PS and PF. The pipe threads I see most on turned brass and aluminum parts are PT and NPT, from 1/8 to 1/2.

What do PT, PS and PF mean on a drawing?

They are the designations used before the Japanese standards adopted the ISO letters. The current standards, JIS B 0203 for taper pipe threads and JIS B 0202 for parallel pipe threads, are based on ISO 7-1 and ISO 228-1 and use R, Rc, Rp and G. PT and PS are still listed in an annex to JIS B 0203, and PF in an annex to JIS B 0202. Each annex says a date for withdrawing them will be set at the next revision.

Under the old name, PT covered both the taper male and the taper female. The current letters separate them: R and Rc. In English-speaking markets, the ISO 7-1 taper thread is usually called BSPT and the ISO 228-1 parallel thread BSPP, the names Swagelok's thread identification guide uses.

Why do 1/2 PT and 1/2 NPT thread together but not seal reliably?

In that size the numbers are almost the same. Both have 14 threads per inch, a pitch of 0.0714 in (1.814 mm). Both have the same taper: Swagelok lists a 1°47′ taper angle for each, which is 1 in 16 on the diameter. At the gauge plane, the pitch diameter of a 1/2 PT is 19.793 mm (0.7793 in). For 1/2 NPT, the pitch diameter at the hand-tight plane (E1) is 0.77843 in (19.772 mm). The 3/4 size is the same story.

What differs is the shape of the thread: 55° with rounded crests and roots against 60° with flat crests and roots. With different angles, the flanks of the two threads cannot lie flat against each other.

Swagelok's identification guide says 1/2 and 3/4 ISO 7-1 and NPT threads "can be difficult to identify because they are very close in design". An article in Power Transmission Engineering states the consequence: the two series can sometimes be threaded together, but "a pressure-tight seal is unlikely and certainly not guaranteed."

I have seen a 1/2 PT and a 1/2 NPT thread together, so a trial fit proves nothing about which thread it is. Measuring the largest diameter with a caliper and checking it against a dimension table gives a first hint. Measuring the thread angle is what settles it.

In the other sizes, counting threads tells them apart

One thread per inch sounds small. Across the 4.1 threads of hand-tight engagement that ASME B1.20.1 gives for a 1/4 NPT, a 19-per-inch thread drifts about 0.012 in (0.3 mm) out of step with an 18-per-inch one.

A parallel British female thread can be one of two threads

G and Rp are both parallel and both 55°, but they are designed for different joints. Rp takes a taper R male thread and seals on the threads with a sealant (ISO 7-1). ISO 228-1 says G threads "are not suitable as jointing threads where a pressure-tight joint is made on the thread". A pressure-tight G joint seals outside the threads, on a gasket or an O-ring pressed between two faces.

In Japanese shop terms, that is the difference between PS and PF. A drawing that says only "1/4 parallel" leaves the choice to the shop, and "1/4 BSPP" is a common name, not a designation from either standard. For an external G thread, add the tolerance class, A or B. ISO 228-1 gives class B twice the tolerance of class A.

Which male thread goes with which female thread?

NPTF, the American dryseal taper thread, and the NPS straight threads such as NPSC and NPSM are not covered here.

To join a 55° thread to a 60° thread, use an adapter that has one of each. I have turned adapters like that, with a PT thread on one end and an NPT thread on the other.

How can you tell them apart on the bench?

This is for a part in hand with no clear callout, such as a sample to copy or a fitting with an unmarked thread.

Taper or parallel first. Swagelok's method is to measure the crest diameter on the first, fourth and last full threads with a caliper. If the diameter grows from the end of a male thread toward its shoulder, or shrinks going into a female thread, the thread is tapered. If all three match, it is parallel.

Then the threads per inch: count the threads in one inch, or divide 25.4 by the pitch in millimeters. Table 2 settles 1/8, 1/4, 3/8 and 1.

Then the angle. Swagelok's guide tries a pitch gauge of each form, 55° and 60°, and sees which one fits. By eye, all I can judge is the crest: a 60° thread looks sharper, a 55° thread looks blunter. For 1/2 and 3/4 a look is not proof. I measure the thread angle on a vision measuring system.

Last, a thread gauge from the right system. An NPT thread is checked with an L1 plug or ring gauge screwed on by hand. ASME B1.20.1 accepts the thread when the end of the part lands "not more than one turn, large or small" from the gauge's basic notch or step. ISO 7 taper and Rp threads are checked with plug and ring gauges to ISO 7-2. That step on the gauge is how I accept NPT threads day to day: the end of the part has to land within the limits marked on the gauge.

What should the drawing and the RFQ say?

Write the standard and the designation, once for each thread:

• 1/4-18 NPT, per ASME B1.20.1
• R 1/4 or Rc 1/4, per ISO 7-1
• Rp 1/4, per ISO 7-1, for a parallel female that mates with an R male
• G 1/4 A or G 1/4 B, per ISO 228-1, for an external G thread; G 1/4 for the internal thread

Not "1/4 pipe", not "1/4 BSP", not "PT 1/4" on its own.

Ask the supplier which gauge will accept each thread. An NPT thread is accepted with an NPT L1 gauge, not a PT gauge. If a drawing or a supplier's quote uses PT, PS or PF, ask which ISO designation it means before the part is cut. It is a one-line question.