BSPP vs BSPT: Parallel vs Tapered British Standard Pipe Threads (Sealing, Identification & Selection)

BSPP (British Standard Pipe Parallel) runs a straight thread. It seals on a face, an O-ring, or a bonded washer. BSPT (British Standard Pipe Taper), by contrast, is the tapered member of the same thread family; it seals on the threads themselves, with a sealant doing the work. Both share the 55° Whitworth thread form. Only BSPT carries the 1:16 taper. Get this distinction wrong, and you're looking at the gap between a leak-free instrumentation joint and a fitting that never quite seals.

This page uses BSPT and BSPP to mean fluid-system and instrumentation pipe threads on stainless and alloy fittings and valves. It doesn't cover domestic water-plumbing fittings, brewing hardware, or camera and tripod threads.

BSPP vs BSPT at a Glance: Parallel vs Taper Comparison

Both threads sit in the British Standard Pipe (BSP) family, and both share a 55° Whitworth flank angle along with nominal-bore sizing. Where they part ways is the taper and where the seal actually forms. The table below lines the two up side by side. The values come from standard published thread-standard facts, and you can adjust them to fit your requirement.

What Is BSP? The 55° British Standard Pipe Thread Family

British Standard Pipe (BSP) covers a family of inch-based pipe threads built on the Whitworth 55° thread form. It's standardized internationally through ISO 228 and ISO 7, and you'll find it used widely across Europe, the Commonwealth, and Asia. BSPP and BSPT are the two members of that family. BSPP is the parallel, or straight, thread; BSPT is the tapered one. Both get sized by nominal bore, the traditional pipe size such as 1/4" or 1/2", rather than by measured thread diameter. That's why a "1/2 BSP" thread actually measures larger than half an inch across. The whole family shares that 55° form, so the parallel and tapered versions look similar at a glance. What separates them is the taper and the sealing method. For the American 60° counterpart and how the two systems compare, see BSP vs NPT threads compared.

What Is BSPP (Parallel / G Thread)?

BSPP stands for British Standard Pipe Parallel, designated G under ISO 228. It's a straight thread, and the crest and root diameters hold steady along the entire length. A parallel thread can't pinch shut against a mating taper, so a BSPP joint never seals on the threads themselves. Sealing happens off-thread instead: a bonded Dowty washer under the hex, an O-ring, a flat gasket face, or a machined port seal like DIN 3852. That's what makes BSPP the go-to choice for reusable, high-integrity connections, plus gauge or manifold ports, where a clean, repeatable metal or elastomer seal counts for more than thread interference. Rp designates the female parallel form that mates with an external taper male. Crestflo offers BSPP as an end connection on its instrumentation fittings. See BSPP (G) end-connection fittings and ISO 228 parallel-thread fittings.

What Is BSPT (Taper / R Thread)?

BSPT stands for British Standard Pipe Taper. The external thread carries the designation R under ISO 7, and it's cut on a 1:16 taper, so the diameter narrows as you move toward the tip. Screw an R male into a taper (Rc) or parallel (Rp) female, and the flanks wedge together, forming a metal-to-metal interference joint. That interference alone rarely makes a perfect seal, though. A thread sealant, either PTFE tape or an anaerobic paste, gets applied to close off the helical leak path. ISO 7 and BS EN 10226 govern BSPT and its internal counterparts; BS EN 10226 replaced the older BS 21. You'll still find tapered pipe threads on legacy equipment, gauges, and process connections built around a self-sealing thread design. Crestflo cuts BSPT to spec as an end connection. See BSPT (R) tapered-thread fittings for details.

BSP Thread Designations Decoded: G, R, Rc, Rp (and PF/PT)

That letter in a BSP callout does real work: it tells you the geometry, plus whether the thread is internal or external. G marks the ISO 228 parallel thread, used for both male and female parallel forms. R is the ISO 7 external taper, while Rc covers the ISO 7 internal taper. Rp, meanwhile, is the ISO 7 internal parallel thread that mates with an R taper male. On older drawings you'll find PF for parallel and PT for taper instead. So, is BSP the same as a G thread? Yes: G is a BSP parallel thread, which makes it compatible with BSPP parallel ports, not with tapered threads or NPT.

A callout with a size like G1/4 or R1/4 packs the nominal bore and the thread system into a single token. Values track the applicable standard, and they can be adjusted to fit whatever the job calls for.

How Each BSP Thread Seals: Face Seal vs Thread Seal

The two threads part company on one thing above all: where the seal actually forms. People mix this up constantly, which is exactly why the table spells it out.

A BSPP joint uses a separate seal element instead of relying on deformed threads, so it can be assembled and taken apart many times without losing integrity. That's a real advantage on instrumentation ports that get opened repeatedly for calibration or service. A BSPT joint, by contrast, puts the seal in the thread flanks themselves. That works well where a dedicated seal face isn't practical, but it's far less forgiving of over-torque, contamination, or repeated make-and-break cycles.

BSP Thread Geometry: 55° Angle and 1:16 Taper

Both BSPP and BSPT share the same Whitworth 55° thread form, complete with rounded crests and roots. Where they part ways is the taper. BSPP runs parallel, with a 0° taper. BSPT, by contrast, is cut to a 1:16 taper, meaning the diameter changes by 1 unit for every 16 units of length, which works out to roughly a 3.58° included angle. This 55° form is also what sets BSP apart from the American National Pipe Thread, which is built on a 60° form; the two won't interchange, even when the nominal size matches. Spotting the difference between parallel and tapered geometry is the first thing you need to master to tell the two BSP threads apart by eye.

BSP Thread Chart: Size and Threads Per Inch (TPI)

BSP threads take their size from the nominal bore, and each size carries a fixed threads-per-inch (TPI) figure. That TPI doesn't change between the parallel (G) and tapered (R) versions at a given size; it's the same number either way. The dimensions listed below are standard, published BSP figures. They follow the applicable standard, but they can be adjusted to suit specific requirements.

Can BSPP and BSPT Mate? Compatibility and Interchangeability

A tapered R (BSPT) male seals into a parallel Rp female or a tapered Rc female per ISO 7. That's the pairing the standard actually calls for. A fully parallel G (BSPP) male tells a different story: it'll start into a tapered female easily enough, but it can't hold a reliable seal there, since parallel threads don't wedge the way a taper does. A BSPP male needs its own face, O-ring, or bonded seal to close the joint. Mix BSPP and BSPT at random and you risk a joint that leaks, cross-threads, or chews up the mating part. Stick to the rule: match the male-female pairing the standard intends, R into Rp or Rc, G into a G/Rp port with a seal element, and don't assume a parallel and a taper thread will seal together just because the nominal size happens to match.

BSP vs NPT: Quick Contrast

BSP and NPT aren't interchangeable. The two use different thread forms: BSP is 55° Whitworth, NPT is 60°, and the pitch and diameters differ as well. A 3/8 BSPT isn't the same as a 3/8 NPT: the size label matches, but the thread angle and pitch don't, so the two will at best partially engage and can gall or damage each other. Crestflo covers the full cross-standard comparison in BSP vs NPT threads compared, the NPT thread standard, ASME B1.20.1, and NPT vs NPTF dryseal.

How to Tell BSPP from BSPT (Thread Identification)

You'll usually be able to tell the two apart with nothing more than your eye and a straightedge. Lay a straightedge, or the flat edge of a caliper, along the thread crests. Parallel crests mean BSPP. Crests that taper toward the end of the thread mean BSPT. To confirm, measure the diameter at the first full thread and again at the last; any real difference between the two points to a taper. A thread pitch gauge settles the rest, confirming the TPI and the 55° form and ruling out a 60° NPT of similar size. For the full method, including pitch gauges, calipers, and every common thread system, check the thread identification guide, along with the broader context in metric vs imperial tube fittings.

Choosing a BSP Thread for Instrumentation Service

For instrumentation, the seal method usually drives the choice. Where a joint has to be reusable, leak-tight and repeatable, gauge ports, manifold connections, analyzer and sampling systems, a BSPP (G) thread with a bonded washer, O-ring or machined face seal tends to win out. It seals off the thread itself and holds up under repeated make-and-break cycles. BSPT (R) makes more sense where legacy equipment or an existing process connection was built around a self-sealing tapered thread.

Either way, don't pick the thread in isolation. Material matters just as much: 316/316L covers general service, while Duplex, 6Mo and high-nickel alloys such as Hastelloy C-276 or Inconel handle corrosive and sour duty. Details on material options for BSP-threaded fittings lay out the tradeoffs. Crestflo carries both thread types across that entire material range. Take a look at instrumentation tube fittings or request a quote to nail down your end connection.

Why Crestflo: BSPP & BSPT Ends, Engineered to Spec

Crestflo manufactures instrumentation fittings and valves, cutting national, international, and custom threads to spec, including BSPP (G), BSPT (R), and ISO-parallel ends, in both imperial and metric sizes. Not a reseller's line. This comparison comes from a company that actually machines both threads.

Fittings span 316/316L through 904L, Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, Alloy 20, the Inconel/Incoloy/Monel/Hastelloy nickel alloys, and Titanium Gr2, with end connections running up to 2" (50 mm) OD. Every part carries a heat-number mark under the EN 10204 3.1 material test certification standard (3.2 available on request). Compression tube fittings, meanwhile, are performance-qualified to ASTM F1387 for drop-in interchange.

Crestflo is the instrumentation division of a four-decade-old export house recognized by the Government of India, certified to ISO 9001 / 14001 / 45001 and PED. The product itself is of Indian origin, though US or European melt is available on request, with full material traceability throughout. Standard items ship from ready stock; specials are made to order, with delivery running 6–8 weeks. We Engineer Confidence.

Want a quote on BSPP or BSPT ends in any alloy? Just ask. Or order a sample first and check the fit yourself:

• Request a quote, BSPP or BSPT ends, any alloy
• Request a sample

Crestflo offers three lines of tube fittings: BSPP (G) end-connection fittings, BSPT (R) tapered-thread fittings, and ISO 228 parallel-thread fittings.

The Crestflo Engineering Team reviewed this document. For US inquiries, reach out to sales@crestflousa.com or call +1 346 594 9005. The office is based in Houston, TX.

Frequently Asked Questions: BSPP vs BSPT

Related Resources

• BSP vs NPT threads compared
• NPT vs NPTF dryseal
• Thread identification guide
• Metric vs imperial tube fittings
• ASME B1.20.1, the NPT thread standard

Browse our BSPP (G) tube fittings, our BSPT (R) tube fittings, and our ISO-parallel tube fittings.

• Instrumentation resources hub

Reviewed by the Crestflo Engineering Team