Two turns of thread do not seal – why NPT and R engage and still do not match

Rule of thumb

An NPT thread engages the first turns in an R or Rp thread – flank angle and sealing principle still do not match.

Where does the rule come from?

NPT (ASME B1.20.1) and R (EN 10226-1) are both taper pipe threads with taper 1:16, and at 1/2 inch both have 14 threads per inch. That is why an NPT nipple can be screwed two or three turns into an Rp socket – and whoever sees that takes the threads for the same. The rule comes from assembly: the joint jams after the first turns, is forced further, holds at the pressure test and drips after weeks. The storeman who knows both threads recognises them by the flank angle – 60° against 55°.

What is technically correct?

The flank angle is the difference the eye does not see: NPT has 60°, the Whitworth pipe threads R, Rp, Rc and G have 55°. With different flank angles the flanks bear only on the edges; the thread does not seal on the flank, deforms when tightened and strips at high torque. In addition the threads per inch differ in other sizes (NPT 3/4: 14, R 3/4: 14; NPT 1: 11.5, R 1: 11). G is parallel and never seals in the thread but on a face with a gasket; Rp is parallel and seals against a taper R. A thread is identified only when form, flank angle, taper and sealing principle match – not when it engages.

Where does the rule end?

• NPTF (dryseal) seals in the thread without sealant but is also 60° – as unsuitable for R as NPT.
• An adapter nipple R to NPT is a normal stock part – the rule does not forbid mixing, it forbids confusing.
• At low pressure and with sealant the mismatch often “holds” – until the first temperature change. Tight it is not.
• Metric fine threads (M) and UNC/UNF are not pipe threads; the rule applies to pipe and fitting threads.

What does the standard say?

EN 10226-1 describes taper external threads R and parallel internal threads Rp, EN 10226-2 taper internal threads Rc – all Whitworth profile 55°, sealing in the thread; ISO 228-1 the parallel G, not sealing in the thread. ASME B1.20.1 describes NPT with 60° profile and taper 1:16, sealing in the thread with sealant. None of the standards names the other as a counterpart; the thread tables of our stock list threads per inch and diameter per size.

DIN/EN versus ASME/ASTM

That is the rule itself: R and Rp are EN world, NPT is ASME world. In projects to ASME specification threaded connections on valves, instruments and hoses are NPT; in the EN world R/Rp or G. An imported device with NPT connections needs the adapter nipple – and that is a separate stock part with both threads, not a “close enough”.

Worked example

A pressure gauge from an ASME project with 1/2 NPT connection is screwed into an Rp 1/2 socket of an EN manifold: two turns easily, then hard, with hemp and force to the stop. Pressure test at 10 bar holds; after the first passage of steam it drips because the 60° flanks have widened the 55° flanks. Correct: adapter nipple Rp 1/2 × NPT 1/2 from stock, two minutes.

Zickwolff reference

No company lore for this rule.

Checked today

Verdict: applies (class A). Flank angle and sealing principle are in the standards; the rule is correct without restriction.

Further reading

• Mixing standards worlds: what fits, what only appears to fit, what does not fit – threads of both worlds in the pairing table
• Standards translator: DN to NPS, PN to Class, identify threads, material counterpart – identify threads from designation or measurements
• Stainless steel threaded fittings: threads, standards and why lengths vary – threaded fittings from stock
• DN is not NPS, PN is not Class – why translating between the standards worlds is only an approximation – the sister rule on DN and PN