ISO 7-1

Covers — pipe threads in the BSPT family where the pressure-tight joint is made on the thread flanks:

Size range — R 1/16 through R 6 (nominally inch-based designations, same numbering as NPS but not dimensionally identical).

Geographic scope — dominant standard in Europe, UK, Australia, Japan (as JIS B 0203), South-East Asia, and on international OEM equipment. Plants sourcing globally must audit thread standards on imported equipment.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Parallel pipe threads where the seal is by separate element (O-ring, washer) → ISO 228-1 (G / BSPP)
• North American taper pipe threads → ASME B1.20.1 (NPT) and B1.20.3 (NPTF)
• European tapered pipe threads (national adoption) → EN 10226-1 (steel pipes) references ISO 7-1
• Japanese equivalent → JIS B 0203 (identical geometry, different designation format)
• Threaded fitting pressure ratings → the applicable fitting standard (e.g., EN 10242 for malleable iron)
• Metric-thread pipe connections → ISO 6149 / DIN 3852 (O-ring port, not pipe thread)

R/Rc and NPT are not interchangeable — identical pipe size numbers (e.g., R ½ and ½ NPT) refer to different thread geometries. Never assume compatibility; always verify the thread standard marked on the fitting.

ISO 7-1 establishes the following minimum requirements for BSPT threads:

Thread Geometry

• Thread angle — 55° included angle (Whitworth form), with rounded crests and roots — distinct from NPT's 60° sharp V-form.
• Taper — 1 in 16, measured on the diameter (equivalent to ¾ in/ft — numerically the same rate as NPT, but the thread form differs).
• Thread form — Whitworth rounded profile. Rounded crests and roots reduce stress concentration but still leave a helical leak path; a sealant is required.

Thread Data by Size

½ and ¾ sizes share TPI with NPT (14), which is a leading cause of cross-threading — the pitch feels similar but the 55° vs. 60° flank angle ensures the joint will not seal properly.

Mating Combinations

• R × Rc — taper external × taper internal: standard combination, progressive wedge on make-up.
• R × Rp — taper external × parallel internal: acceptable per ISO 7-1 for moderate pressures; less engagement depth available.

Tolerances and Gauging

• Pitch diameter tolerances are defined per size for both R (external) and Rc/Rp (internal) threads.
• Gauging uses step gauges to verify the reference plane (equivalent to NPT's L1 check) and effective thread length.
• Standard reference gauge lengths are tabulated in ISO 7-1 Annex A.

Designation Format

• External taper: R ½ (or R 1/2)
• Internal taper: Rc ½
• Internal parallel: Rp ½
• Old British designation (still found on legacy equipment): ½ BSP or ½ BSPT

The BSPT Family and How It Works

ISO 7-1 defines the BSPT (British Standard Pipe Taper) family — the pressure-thread standard for pipe systems outside North America. Like NPT, it seals on the thread flanks by wedging a taper-on-taper assembly, but the 55° Whitworth profile (with rounded crests and roots rather than a sharp V) gives it a distinctly different feel and sound during make-up.

The helical leak path between the rounded crests and roots means BSPT requires a thread sealant: PTFE tape (2–3 turns) or anaerobic pipe dope. The standard does not define a dryseal variant — for contamination-critical applications, users typically switch to an O-ring face seal (ISO 228-1 G thread with an O-ring groove, or SAE J1926/DIN 3852 port connections).

Thread Type Summary

Why R and NPT Cannot Be Interchanged

The ½″ size is the most dangerous trap: identical TPI (14) makes the threads appear to engage smoothly, but the 5° flank angle difference means the flanks never seat on their mating faces — the joint will leak or seize unpredictably.

Regional and National Equivalents

Legacy equipment marked "BSP" or "BSPT" (no ISO number) conforms to BS 21, which is dimensionally identical to ISO 7-1 — the threads are interchangeable.