BSP Thread Gauge Guide:
BSPP, BSPT, G and R Thread Inspection

BSP (British Standard Pipe) threads are the most widely used pipe thread standard outside North America — covering the UK, Europe, Australia, the Middle East, Asia, and most of the rest of the world. This guide explains BSPP (G threads) and BSPT (R threads), size charts, gauge types, and how to select the correct thread gauge for each BSP standard.

Need gauges for this? GageTools makes BSPP and BSPT (G and R) thread plug and ring gauges with MOQ 1 piece: plug gauges from USD 20, ring gauges from USD 30, and standard stock sizes ship within 72 hours. Send your thread size for a quote.

Need gauges for this? GageTools manufactures GO/NO-GO pipe thread plug and ring gauges (NPT, NPTF, BSPP and BSPT) in Dongguan, China. Standard sizes ship from stock within 72 hours, MOQ is 1 piece, and every RFQ gets a reply within 24 hours — request a quote.

What Is a BSP Thread?

BSP stands for British Standard Pipe. BSP threads use a 55° thread angle — different from the 60° angle used by metric, UN, and NPT threads. This single characteristic makes BSP threads immediately incompatible with metric or NPT connections, even when the nominal size appears similar.

BSP threads are used in plumbing, hydraulics, pneumatics, compressed air systems, oil and gas equipment, and general pipe fittings across most of the world outside North America.

BSPP vs BSPT — Key Differences

BSP threads are divided into two main types — parallel (BSPP) and taper (BSPT). They use the same 55° thread angle and the same TPI for each nominal size, but they differ in form and sealing method. Using the wrong gauge type will produce incorrect inspection results.

BSP thread plug gauges (left) for internal threads and ring gauges (right) for external threads. BSPP and BSPT gauges are not interchangeable.

ISO 7 Designation — R, Rc, Rp Explained

R — External taper thread (male). Mates with Rc or Rp internal thread.

Rc — Internal taper thread (female). Mates with R external taper thread.

Rp — Internal parallel thread (female). Mates with R external taper thread. Note: Rp is parallel, NOT taper — do not confuse with G thread.

G — Parallel thread (both internal and external). BSPP under ISO 228. Requires separate sealing.

BSPP Thread Size Chart — G Thread (ISO 228)

Standard: BS EN ISO 228-1. Gauge standard: ISO 228-2. All thread dimensions in millimetres. Parallel form — both internal and external threads are straight.

BSPT Thread Size Chart — R / Rc Thread (ISO 7)

Standard: ISO 7-1. Gauge standard: ISO 7-2. Taper 1:16 on diameter. Dimensions at gauge plane (reference position) in millimetres.

BSP vs NPT — Why They Are Not Interchangeable

BSP and NPT are frequently confused because they share the same nominal size designations and some sizes have the same TPI. However, they are fundamentally incompatible because of the different thread angles — 55° for BSP vs 60° for NPT.

Property	BSP	NPT
Thread angle	55°	60°
Standard	ISO 228 / ISO 7	ASME B1.20.1
1/2″ TPI	14	14
Same TPI = compatible?	NO — thread angles differ
Primary region	UK · EU · AUS · Asia	USA · Canada
Interchangeable?	NEVER — always confirm standard

⚠️ Never assume BSP and NPT are the same because the nominal size or TPI matches. The 5° difference in thread angle means the threads will appear to engage but will not seal correctly and may leak or damage components.

Need gauges? See our BSP thread gauges: G, R and Rc plug and ring gauges and request a quote.

How to Select the Correct BSP Thread Gauge

How to Identify BSP vs NPT on a Part

BSP and NPT threads in the same nominal size look almost identical, and several sizes share the same thread count. Mixing them up is one of the most common causes of leaking fittings and rejected parts in export trade. Use three checks together: threads per inch, flank angle and taper.

Step 1: Count threads per inch

Use a thread pitch gauge or count crests over one inch. The table below compares standard values for common sizes. Where the TPI matches (1/2, 3/4), the count alone cannot separate the two systems.

Step 2: Check the flank angle

BSP uses the Whitworth form with a 55° included angle and rounded crests and roots. NPT uses a 60° included angle with flat (truncated) crests and roots. A 55° and 60° screw pitch gauge held against the thread under a light shows the difference quickly: the wrong blade leaves a visible gap at the flanks.

Step 3: Check for taper

NPT is always tapered (1:16 on diameter). BSP can be parallel (G, Rp) or tapered (R, Rc, also 1:16). If the thread is parallel, it is not NPT. If it is tapered, the flank angle and TPI decide between BSPT and NPT. For a final answer on production parts, use the correct GO/NO-GO or taper gauge rather than hand tools.

Sealing Methods: Parallel vs Tapered Pipe Threads

The sealing method is the practical reason why the thread designation matters. The thread type determines where the seal is made and which gauge verifies the joint.

A G thread has no interference, so it cannot seal on its own threads, no matter how tightly it is torqued. A tapered R or NPT thread relies on flank interference plus sealant. Screwing an NPT male into a BSPT female may engage a few turns but leaves a leak path because the flank angles and pitches do not match.

G, R, Rp and Rc Designations Explained

ISO standards replaced the older “BSPP” and “BSPT” trade names with letter codes. Drawings from European and Asian customers normally use these codes, so the gauge order must follow them exactly.

• G (ISO 228-1): parallel pipe thread where pressure-tight joints are not made on the threads. External G threads have tolerance class A or B (for example G 1/2 A); internal G threads have a single tolerance class.
• R (ISO 7-1): taper external thread, sealing on the thread.
• Rc (ISO 7-1): taper internal thread, mates with R.
• Rp (ISO 7-1): parallel internal thread designed to mate with a taper R external thread and seal on the thread. It is not the same as an internal G thread.

An R/Rc joint gives the most reliable seal; an R/Rp joint is also permitted by ISO 7-1. Never assume Rp and G are interchangeable because both are parallel: the tolerance zones and purpose differ.

Which Gauges to Use for Each BSP Thread

G limit gauges work like metric GO/NO-GO gauges: the GO member must assemble and the NO-GO member must not. Taper gauges per ISO 7-2 work differently: the gauge is screwed on or in by hand and the position of the part’s end face relative to the step or notch shows whether the thread diameter at the gauge plane is within tolerance. When ordering, state the full designation (for example G 3/4 B or Rc 1/2), whether you need plug or ring, and whether an ISO/IEC 17025 calibration certificate is required. For a side-by-side of NPT, NPTF, BSPP and BSPT gauges, see our pipe thread gauge comparison, and for sizes not in stock we can make custom gauges from your drawing.

Frequently Asked Questions