Pipe Threads Explained: NPT, BSPT and BSPP and Why They Must Not Be Mixed

Threaded joints are the oldest and still the most common way of connecting small-bore pipe, valves, instruments and equipment. They are also the source of a steady trickle of leaks that trace back to one cause: two threads of different families screwed together because they looked alike and felt tight. Saudi sites see all three major families, American NPT on imported equipment and instruments, British-origin BSPT on the galvanised and malleable iron pipework that dominates building services, and parallel BSPP on hydraulic, pneumatic and some valve connections. This article explains each, shows where they coincide and sets out how to keep them apart on a purchase order and in a store.

Three families, two standards bodies

NPT, the American National Pipe Taper thread, is defined in ASME B1.20.1, Pipe Threads, General Purpose (Inch). It has a 60 degree thread angle and a taper of 1 in 16 on diameter, which is 3/4 in per foot or an included half-angle of 1 degree 47 minutes. Both the external and the internal thread are tapered, and the seal is made by the flanks of the threads wedging together as the joint is tightened. Because the crests and roots do not fully close, a sealant or PTFE tape is needed for a leak-free joint.

BSPT and BSPP share the 55 degree Whitworth thread form inherited from the British standard BS 21 and are now defined internationally. ISO 7-1, Pipe threads where pressure-tight joints are made on the threads, Part 1: Dimensions, tolerances and designation, covers the sealing threads: R is the tapered external thread, Rc the tapered internal thread and Rp the parallel internal thread that mates with R. The taper is again 1 in 16. ISO 7-1 is adopted in Europe as EN 10226-1 and the sizes run from 1/16 to 6 in. ISO 228-1, Pipe threads where pressure-tight joints are not made on the threads, covers the parallel G thread: both male and female are parallel, and the title tells you the essential point, which is that the thread itself is not a seal.

Where the threads coincide and where they do not

The dangerous overlap is in pitch. The threads-per-inch (TPI) values for each nominal size are:

At 1/2 in and 3/4 in, the two most common sizes on a building site, NPT and BSP have the same pitch. A 1/2 in NPT male will screw several turns into a 1/2 in BSPT female and feel engaged; the 60 and 55 degree flanks then meet on a line rather than a surface, the joint takes sealant badly and it weeps under pressure or vibration, often weeks later. At 1/8, 1/4 and 3/8 in the pitches differ by one thread per inch, which is enough to bind after a turn or two; at 1 in and above the half-thread difference gives the same result. Only the 1/2 and 3/4 in sizes cross over quietly, which is exactly why they cause most of the trouble.

Parallel G threads introduce the second kind of mismatch. A G male will enter an Rp female freely because both are parallel with the same pitch and angle, but nothing seals: the G thread is designed to seal on its shoulder against a washer or O-ring, and the Rp fitting has no seat for one. Conversely an R tapered male driven into a G female will jam a few turns in and may split the female fitting. The thread name on the drawing, not the feel in the hand, decides the match.

How each joint actually seals

NPT and BSPT joints are made up by hand, then wrenched a defined number of turns past hand-tight; the taper does the work and the sealant fills the spiral leak path at the crests. PTFE tape wound in the direction of tightening or an anaerobic or paste sealant rated for the fluid is standard. Over-tightening a tapered joint into a cast fitting splits the fitting; under-tightening leaves the flanks loose. Steel pipe is threaded on site with dies that must match the fitting standard, so a project that mixes NPT valves with BSPT fittings needs transition nipples, not a second set of dies in the same box.

BSPP joints are made up to a shoulder. The male has a flat face or an undercut, and a bonded seal (a metal washer with a rubber insert), an O-ring in a groove or a soft washer is compressed between the male shoulder and the female face. The threads only provide the clamping force, which is why G threads are the standard on hydraulic adaptors, gauges and components that are assembled and disassembled repeatedly: the seal is renewable and the threads are never damaged by sealant or over-tightening. Dryseal variants of the American threads, which seal metal-to-metal without sealant, exist for hydraulic and refrigeration work; refer to the relevant ASME standard before specifying them.

Identifying threads on site

Three tools settle any argument: a thread pitch gauge, a calliper and a thread angle template. Measure the TPI first; a reading of 19 or 28 means BSP, 18 or 27 means NPT. For 14 TPI, check the angle, 55 against 60 degrees, with a template or by trying known reference fittings. Then check for taper by laying a straight edge along the crests or by measuring the diameter at both ends of the thread. Markings help: American fittings are commonly stamped NPT or carry the ASME designation, European fittings carry R, Rc, Rp or G with the size. Fittings with no markings and no documentation should be assumed to be whatever the cheapest supplier in their origin market uses, which is a reason to buy from a source that states the thread standard.

Keeping threads apart in procurement

• Write the thread standard and designation on every line of the purchase order: "1/2 in NPT to ASME B1.20.1", "Rc 1/2 to ISO 7-1", "G 1/2 to ISO 228-1".
• Specify the thread on valves, strainers and instruments as well as on fittings; imported equipment is the usual source of NPT on a BSPT site.
• Buy transition nipples (NPT one end, BSPT the other) as a stock item rather than improvising.
• Store NPT and BSP fittings in separate, labelled bins, and keep the dies for each standard separately marked.
• Make the thread standard part of goods inspection: a pitch gauge at the receiving bay is cheaper than a leak behind a ceiling.
• Where BSPP is used, order the matching bonded seals or O-rings with the fittings.

Key takeaways

• NPT (ASME B1.20.1) is a 60 degree tapered thread; BSPT (ISO 7-1, R/Rc/Rp) is 55 degree tapered; BSPP (ISO 228-1, G) is 55 degree parallel.
• NPT and BSPT seal on the thread flanks with sealant; BSPP seals on a face with a washer or O-ring.
• 1/2 in and 3/4 in NPT and BSP share 14 TPI and will screw together, but the different flank angles mean the joint is not reliable.
• G threads never seal on the thread, so a G male in an Rp female leaks even though it fits.
• Identify threads with a pitch gauge and angle check; buy fittings and equipment with the thread standard stated on the order.

Frequently asked questions

Can I use PTFE tape to make an NPT and BSPT joint seal?

Sometimes it will hold at low pressure for a while, which is why the practice persists. The 55 and 60 degree flanks do not bear evenly, the joint relies entirely on the tape and it fails under thermal cycling or vibration. Use an adaptor that has the correct thread on each end.

Is BSPP the same as BSP?

BSP is a family name covering both the tapered BSPT (R, Rc) and the parallel BSPP (G) threads. They share angle and pitch but seal in completely different ways, so an order that says only "BSP" is incomplete.

Which thread will my GI and MI fittings have?

Malleable iron fittings sold in the Gulf are overwhelmingly threaded to ISO 7-1 (BSPT), with Rc or Rp internal threads. Fittings from American suppliers or on American equipment are NPT. Check the standard on the data sheet or the marking on the fitting rather than assuming.

How Al Safsaaf can help

Al Safsaaf Trading & Contracting Company supplies GI nipples, MI threaded fittings and the wider GI and MI fittings range, together with forged steel threaded fittings and stainless steel threaded fittings. State the thread standard on your enquiry and we will confirm it line by line before quoting; contact us to start.