Guide
BSPP vs BSPT: which pipe thread should your port drawing call out?
BSPP vs BSPT is the choice between two British-origin pipe threads that share the same 55° Whitworth form and the same nominal sizes but seal in opposite ways. BSPP is the parallel G thread of ISO 228-1, which holds the parts together while a washer, O-ring or face seal does the sealing; BSPT is the taper R, Rc and Rp family of ISO 7-1, which seals on the thread itself with a jointing compound.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and buyers specifying ports and fittings on valve bodies, manifolds, regulators, pump housings and instrument fittings that will be sold in Europe, the UK, Australia or Asia, or that must mate with equipment from those markets. It covers how each thread seals, how to tell them apart on a part, how to write the callout so there is only one way to read it, and what happens when either one meets an NPT fitting. The letters BSPP and BSPT do not appear in the current ISO standards, so the first rule of this guide is simple: write the ISO letters on the drawing.
On this page
BSPP vs BSPT: where does each joint actually seal?
A BSPT joint seals on the thread flanks, because the taper wedges the male into the female and a compound or tape fills what is left. A BSPP joint does not seal on the thread at all; the thread only clamps a separate seal against a face or in a groove.
The title of each standard says it plainly. ISO 7-1 covers pipe threads “where pressure-tight joints are made on the threads”, and ISO 228-1 covers pipe threads “where pressure-tight joints are not made on the threads”. Everything else follows from that sentence.
On a taper joint the male R thread runs into the female until the flanks bind. The tighter you turn it, the harder the flanks bear, which is why a taper joint makes up fast and seals without extra parts. The cost is that the joint is set by how far it turns, so the final orientation of an elbow or a sensor head is wherever the thread stops. Each remake pushes the male a little deeper, and a taper wedged into a thin cast boss puts hoop stress into the wall, so on cast aluminum and zinc bodies a taper port needs generous wall around it, or a parallel port instead.
On a parallel joint the G thread carries the axial load and the seal sits somewhere else: a bonded washer squeezed between a hex shoulder and a flat spot face, an O-ring captured in an undercut on a stud end, or a metal-to-metal edge biting into the face. ISO 1179 is the series that standardizes these ports and stud ends for fluid power and general use with ISO 228-1 threads. Because the thread does not seal, the joint can be made and broken many times, and a banjo or adjustable fitting can be set to any angle before tightening.
The practical decision rule: choose BSPT (R into Rc, or R into Rp) for plumbing-style connections that are made once and sealed with compound, and choose BSPP (G with a seal) for hydraulic, pneumatic, instrument and service ports that will be opened again or must be oriented.
What sizes and pitches do G, R, Rc and Rp threads use?
Both standards use the same nominal sizes, the same pitch at each size and the same 55° rounded profile, so a G thread and an R thread of the same size look almost identical. The difference is that the R and Rc diameters change along the length on a 1 in 16 taper, while G and Rp diameters stay constant.
The nominal size is a trade size inherited from old pipe bores, not a measured diameter. A G 1/2 thread has a major diameter close to 21 mm per ISO 228-1, not half an inch, which is the single most common reason a newcomer orders the wrong tap. The table below lists the common sizes. For R and Rc threads, the major diameter applies at the gauge plane, and the basic gauge length is the distance from the small end of the male to that plane.
| Nominal size | Threads per inch | Pitch (mm) | Thread height (mm) | Major diameter (mm) | ISO 7-1 basic gauge length (mm) |
|---|---|---|---|---|---|
| 1/8 | 28 | 0.907 | 0.581 | 9.728 | 4.0 |
| 1/4 | 19 | 1.337 | 0.856 | 13.157 | 6.0 |
| 3/8 | 19 | 1.337 | 0.856 | 16.662 | 6.4 |
| 1/2 | 14 | 1.814 | 1.162 | 20.955 | 8.2 |
| 3/4 | 14 | 1.814 | 1.162 | 26.441 | 9.5 |
| 1 | 11 | 2.309 | 1.479 | 33.249 | 10.4 |
| 1 1/4 | 11 | 2.309 | 1.479 | 41.910 | 12.7 |
| 1 1/2 | 11 | 2.309 | 1.479 | 47.803 | 12.7 |
| 2 | 11 | 2.309 | 1.479 | 59.614 | 15.9 |
What do G, G A, G B, R, Rc and Rp mean in a drawing callout?
G is the parallel thread of ISO 228-1, and an external G thread also needs a tolerance class letter, A or B. R is a taper external thread, Rc a taper internal thread and Rp a parallel internal thread, all per ISO 7-1.
Per ISO 228-1, internal G threads have a single tolerance class, so “G 1/2” on a port is complete. External G threads come in class A and class B, where class B has the wider pitch diameter tolerance. Class A is the safer default for a stud end that must seat squarely on a seal; class B is acceptable for clamping threads such as lock nuts and hose tails where looser fit does no harm. A callout of “G 1/2” on a male part is incomplete, and the shop will either ask or guess.
Per ISO 7-1, the taper family pairs as R into Rc (taper into taper) or R into Rp (taper male into a parallel female). Rp is not the same as G. ISO 7-1 tolerances an Rp thread plus and minus about the basic size and has it checked with the ISO 7-2 taper plug gauge, while ISO 228-1 tolerances an internal G thread from the basic size upward only. ISO 7-1 also says that a G external thread in an Rp internal thread needs special consideration and may not necessarily give a leak-tight joint. In Europe, EN 10226-1 covers the R-into-Rp pairing and EN 10226-2 the R-into-Rc pairing; they are aligned with ISO 7-1, and older UK drawings may still cite BS 21 for the same threads.
Callouts that leave nothing to interpret look like this:
- Gasket or O-ring port: “G 1/2 per ISO 228-1, spot face and seal surface per detail”, with the spot-face diameter, flatness, perpendicularity to the thread axis and surface roughness called out on the detail.
- Male stud end: “G 1/4 A per ISO 228-1”, plus the thread undercut and the shoulder that loads the seal.
- Taper female port: “Rc 3/8 per ISO 7-1, gauge per ISO 7-2”.
- Parallel female for a taper male: “Rp 3/8 per ISO 7-1” or “Rp 3/8 per EN 10226-1”.
- Taper male: “R 3/4 per ISO 7-1”.
- Left-hand thread: add LH, for example “G 1/2 A LH per ISO 228-1”. Without LH, both standards assume a right-hand thread.
Drawings from Japan, Korea, Taiwan and China sometimes still use the old JIS letters: PT for a taper thread, PS for a parallel internal thread to suit a taper male, and PF for the parallel fastening thread. Today JIS B 0203 uses R, Rc and Rp and JIS B 0202 uses G, matching the ISO letters. If a legacy print says PT 1/4, translate it to Rc 1/4 or R 1/4 before it goes to a new supplier, and do not leave it as “1/4 BSP”, which names a size and a family but not a thread.
How do you tell BSPP from BSPT on a part in hand?
Pitch and profile will not separate them, because both use the same threads per inch and the same 55° form. Look for taper on the thread and for a seal face or captured seal on the part.
Male threads. Measure the major diameter over the first full thread and over the last full thread with a micrometer. A G thread reads the same at both ends. An R thread grows along its length: per ISO 7-1, the taper is 1 in 16 on diameter, which works out to 0.625 mm of diameter for every 10 mm of length, and a micrometer shows that easily. A hex with a flat, machined underside and a relief groove at the thread runout is a sign of a G stud end; residue of tape or compound on the threads is a sign of an R thread.
Female threads. A G port usually has a spot face or counterbore around it for a washer or O-ring. An Rc port usually has only a countersink. To confirm, measure the bore at the thread crests near the mouth and again deeper in the hole: an Rc port gets smaller with depth on the same 1 in 16 taper, while a G or Rp port does not. Telling an Rp port from a G port on a finished part is not practical by eye; that is settled by the drawing and the gauge. Per ISO 7-2, both Rc and Rp ports are checked with a 1:16 taper thread plug gauge whose step marks the allowed position of the port face, while a G port is checked with the parallel limit gauges of ISO 228-2.
Then rule out NPT. If the flank angle looks sharper under a loupe, the crests show flats instead of rounds, or the pitch at any size other than 1/2 and 3/4 does not match the table above (NPT has 27 threads per inch at 1/8, 18 at 1/4 and 3/8, and 11.5 from 1 to 2), it is probably NPT. A 55° and a 60° screw pitch gauge leaf held against the thread make the difference visible. The NPT vs BSPT guide covers that comparison in detail.
Can you mix BSPP, BSPT and NPT fittings on one part?
Mix only the pairings the standards allow: R into Rc, R into Rp, and G into G with a seal. Anything else, including a G male in an Rc port and any BSP thread against an NPT thread, should be treated as a defect, not a fitting choice.
R male into a G port. It will often thread in and stop, because the profile and pitch match. But a G port is made to G limits, not Rp limits: it is never checked with the ISO 7-2 taper plug, and at small sizes such as 1/8 its upper limit is larger than the largest Rp the standard allows. The taper may then seal on only a few threads, and the joint may weep under pressure or crack a thin boss as it is tightened harder to stop the leak.
G male into an Rc port. The parallel male binds after a few turns in the tapered female and never reaches the spot face, so the washer or O-ring is not loaded. It looks assembled and leaks.
BSP against NPT. The flank angles differ (55° against 60°) and so do most pitches. Sizes 1/2 and 3/4 share 14 threads per inch in both systems, so those pairs thread together and can even be tightened, which is exactly why they cause leaks that surface at pressure test or in service. Sealant does not turn a mismatched thread form into a sealing joint.
When one product family has to serve North America and BSP markets, make the port thread a controlled variant: two part numbers or a dash number, each with its own thread callout and its own gauge list, rather than one drawing with a note. For North American parallel ports the analog of ISO 1179 is the O-ring boss port of ISO 11926 and SAE J1926, which uses a UN straight thread, so a “parallel port” in a design spec still needs the thread system named. The NPT vs NPTF guide covers the American taper side.
What should the drawing and RFQ say about a BSP port?
Name the full ISO designation, the tolerance class on any external G thread, the mating part and how the thread will be gauged. Then ask the supplier, in writing, whether the right gauge set is already on the shelf.
A complete BSP port package covers five things. The thread: G, R, Rc or Rp, size, class, hand and standard. The depth: for a G or Rp port, full-thread depth and drill depth as for any tapped hole; for an Rc port, the gauge position and the minimum usable thread length from ISO 7-1, not an arbitrary depth. The seal geometry for a G port: spot-face diameter and depth, flatness, perpendicularity to the thread axis and surface roughness where the washer or O-ring lands, because that face, not the thread, holds the pressure. The finish sequence: whether the thread is gauged before or after plating or anodizing, and whether it is masked. The inspection: which gauges accept the thread (ISO 228-2 limit gauges for G, ISO 7-2 gauges for R, Rc and Rp), and whether the first-article report must list the gauge result per port.
Common mistakes to catch at drawing review: “1/2 BSP” with no G or R; “G 1/4” on a male part with no class; an Rc port on a part that will be serviced and resealed often; a G port with no spot face or seal detail; a taper port drawn in a thin cast wall; and a single drawing that says “NPT or BSP to suit market”.
On Saola parts, threaded ports on round bodies and fittings are made on CNC turning, and ports in die-cast and sand-cast housings are made on CNC milling. Both capability pages list ANPT and NPT to ANSI/ASME B1.20.1 as the thread standard, and the turning page states that those threads are gauge-checked and that thread fit is checked with go/no-go gauges before a lot ships. ISO 228 and ISO 7 threads are not on that published list, so put the BSP callout and its gauge standard on the RFQ, and the quote will answer the gauge question before anything is cut. The turning page also notes that a custom thread form means a custom gauge set, a cost that shows up in the unit price rather than as a separate line. Every toleranced dimension on the drawing is measured on the first article, per the quality and inspection page.
Frequently asked
Is BSPP the same as G thread?
Yes. BSPP is the trade name for the parallel pipe thread now defined in ISO 228-1 and designated G. Older documents cite BS 2779 for the same thread. Use the G designation and the ISO number on new drawings.
Is Rp the same as BSPP?
No. Rp is a parallel internal thread in the sealing family of ISO 7-1, with limits set so a taper R male seals in it. G is the non-sealing parallel thread of ISO 228-1. They share profile and pitch, and a shop that treats them as interchangeable can cut a port that looks right and leaks.
Can a BSPT male screw into a BSPP port?
Physically it often can, but it is not a designed joint. A G port is made and gauged to G limits rather than Rp limits, so the taper may seal on only a few threads. Use an R male into an Rc or Rp port, or a G male with a seal into a G port.
Do I use PTFE tape on BSPP threads?
Not as the seal. A G joint seals at the washer, O-ring or metal seal face, and tape on the thread can stop the shoulder from seating and load the seal unevenly. Tape or compound belongs on taper R threads.
Does plating change how a BSP thread gauges?
It can. Coating builds on the flanks. On an Rc or Rp port it makes the taper plug gauge stop short of its step, and on an external G thread it can stop the GO ring gauge from going on. State on the drawing whether gauging happens before or after finishing. The threads after plating guide covers allowances, and the finishes our partner plants apply are listed on the surface finishing page.
Sources
- ISO 228-1:2000, Pipe threads where pressure-tight joints are not made on the threads — Part 1: Dimensions, tolerances and designation, ISO, 2000
- ISO 7-1:1994, Pipe threads where pressure-tight joints are made on the threads — Part 1: Dimensions, tolerances and designation, ISO, 1994
- ISO 7-2:2000, Pipe threads where pressure-tight joints are made on the threads — Part 2: Verification by means of limit gauges, ISO, 2000
- BS EN 10226-1:2004, Pipe threads where pressure tight joints are made on the threads — Taper external threads and parallel internal threads, listing on the New Zealand Government Building CodeHub, accessed October 2026
- ASME B1.20.1-2013 (R2018), Pipe Threads, General Purpose, Inch, ASME, 2013, reaffirmed 2018