NPT vs Straight Threads: How Pipe Threads Actually Seal

Trade size is not the thread size

The first thing to unlearn is that a pipe thread's name describes its diameter. It does not. Pipe trade sizes are legacy references to the approximate bore of the pipe the fitting suits, and they have almost nothing to do with the outside diameter of the thread.

A 1/2 inch NPT thread has a major diameter of 21.34 mm, which is about 0.84 inch. A 1/4 inch NPT measures 13.72 mm across the crests. A 1/8 inch NPT measures 10.29 mm, which is larger than an M10 machine screw. If you measure a fitting and go looking for a thread of that nominal size you will not find one - you have to match the measurement against the pipe tables instead.

This is a recurring cause of misidentification, and it is worth checking the actual dimension every time. Each pipe size page on ThreadRef lists the real major diameter, pitch and TPI alongside the trade size for exactly this reason.

How a tapered thread seals

NPT and BSPT both taper at 1 in 16 on the diameter - three quarters of an inch per foot of length, or 1/16 inch of diameter per inch of thread. As the male thread is screwed into the female, that taper forces the flanks of the two threads into progressively harder contact, and it is the flank interference, not the crest or the root, that forms the seal.

In a standard NPT joint the crests and roots do not meet. The truncation of the profile leaves a small clearance that runs continuously along the helix - a spiral leak path straight through the joint. This is why standard NPT is assembled with PTFE tape or a pipe-thread sealant: the compound fills that helical gap. Sealant on an NPT joint is not a belt-and-braces precaution, it is part of the design.

The Dryseal variant, NPTF, addresses this by specifying a profile in which the crests deform into the roots on assembly, crushing the leak path closed. It can seal without compound, at the cost of being a one-assembly joint in practice - the crests are plastically deformed, so repeated make-up degrades the seal.

Because a tapered joint tightens progressively, there is no fixed torque figure for it. Make-up is specified in turns: hand-tight, then a defined number of additional turns with a wrench, typically two to three for small sizes and varying with size, material and sealant. Over-tightening does not improve the seal; it splits female fittings, which is one of the most common failures in pipework.

• NPT and BSPT taper at 1 in 16 on the diameter.
• The seal forms on the thread flanks, not the crests.
• Standard NPT needs tape or dope to close the spiral clearance.
• Make-up is by turns past hand-tight, never by a bolt-style torque figure.

How a straight thread seals

BSPP - designated G, and defined by ISO 228-1 - is a parallel thread. It measures the same diameter along its entire length, so screwing it in tighter creates no radial interference and no seal. The thread's only job is clamping force.

The actual seal is made on a separate face, and the arrangement varies: a bonded seal washer under the fitting shoulder, an O-ring seated in a machined boss, or a soft copper or fibre washer. The face is what has to be clean and undamaged; a perfect thread with a nicked sealing face still leaks, and a scruffy thread with a good face usually does not.

This makes straight-thread ports the preferred choice in hydraulics and in anything that is assembled and disassembled repeatedly. There is nothing to deform, so the joint can be broken and remade without losing sealing capability, and the fitting can be clocked to a required orientation and then locked down against the seal - something a tapered thread cannot do without either loosening or over-tightening.

Because BSPP is not cut into a tapered hole, ThreadRef publishes no tap drill for it. The internal thread is produced by cutting a parallel thread into a bored hole, not by tapping a drilled hole to a taper.

NPT, BSPT and BSPP side by side

The three families sit close enough to be dangerous. NPT is the American tapered thread with a 60° included flank angle and flat crests and roots, defined by ASME B1.20.1. BSPT is the British tapered thread, designated R and defined by ISO 7-1, with the Whitworth 55° flank angle and rounded crests and roots. BSPP is the British parallel thread, designated G and defined by ISO 228-1, with the same 55° Whitworth form.

At several trade sizes NPT and BSPT even share a thread count. Both 1/2 inch threads run at 14 TPI, and both 3/4 inch threads run at 14 TPI, so a pitch gauge alone will not distinguish them. What does distinguish them is the flank angle - a 55° gauge leaf will not seat on a 60° thread - and the major diameter: 21.34 mm for 1/2 inch NPT against 20.955 mm for 1/2 inch BSPT.

The mismatched combinations feel like they work, which is the trap. A BSPT male will start into an NPT female and go several turns before the 5° flank difference stops it. The joint appears made up and will hold pressure briefly, but the flanks are in point contact rather than face contact, the female fitting has been deformed, and the seal fails in service - often after the assembly has been signed off.

A BSPP male in a tapered female is worse, because it can be run in until it jams and still be leaking. There is no combination of tape and torque that makes it right.

• NPT: 60° flank angle, tapered, flat crests - ASME B1.20.1.
• BSPT (R): 55° Whitworth, tapered, rounded crests - ISO 7-1.
• BSPP (G): 55° Whitworth, parallel, gasket-sealed - ISO 228-1.
• 1/2 in NPT and 1/2 in BSPT are both 14 TPI - the pitch will not tell them apart.

Assembly practice that avoids leaks

Identify before you assemble. Measure the male thread's major diameter, check the pitch, and check the taper by measuring across the first full thread and again several threads along. A tapered thread will read visibly larger at the back; a parallel one will not. Then confirm the flank angle with a 55° and a 60° gauge leaf if there is any doubt about the family.

Apply sealant correctly on tapered joints. PTFE tape is wrapped in the direction the fitting will turn - clockwise as viewed from the open end for a right-hand thread - so that make-up tightens rather than unwinds it. Start a thread or two back from the leading edge so shreds are not pushed into the system. Anaerobic pipe sealants and non-hardening pipe dopes are alternatives; on stainless, a sealant with PTFE also helps against galling.

Do not chase a leak with more torque. If a tapered joint weeps, break it, inspect both threads for damage, re-dress the sealant and remake it. Continuing to tighten a joint that is already at full flank engagement is how cast fittings crack - usually some time after assembly, under pressure.

And keep the two mental models separate. On a tapered joint, the thread is the seal. On a straight-thread joint, the face is the seal and the thread is only a clamp. Almost every pipe-thread mistake is one of those two models being applied to the wrong fitting.