BSPT Pipe Thread (ISO 7-1 / R Series)
Direct Answer
BSPT (British Standard Pipe Taper), designated as the 'R' series under ISO 7-1 and EN 10226, is a 55° Whitworth pipe thread with a 1:16 taper rate. Commonly used in European, Commonwealth, and Japanese plumbing systems, external tapered R threads mate with either internal parallel Rp threads or internal tapered Rc threads to achieve pressure-tight joint sealing.
Source: ISO 7-1:1994 (EN 10226-1 / BS 21)
Specs
| Standard Governing Body | ISO 7-1 / EN 10226-1 / BS 21 / DIN 2999 |
| Thread Designation | R (Male taper), Rc (Female taper), Rp (Female parallel) |
| Thread Form & Flank Angle | 55° Whitworth with radiused crests and roots |
| Taper Ratio | 1:16 (0.75 in/ft or 1° 47' with centerline) |
| Common Pitches (TPI) | R 1/8": 28 TPI | R 1/4"-3/8": 19 TPI | R 1/2"-3/4": 14 TPI | R 1"-2": 11 TPI |
| Sealing Mechanism | Metal-to-metal flank wedging + jointing paste/PTFE tape |
| Primary Geographical Region | Europe, United Kingdom, Japan (JIS B 0203), Commonwealth |
The ISO 7-1 R, Rc, and Rp System
The ISO 7-1 standard specifies three distinct thread types that work together:
- $R$ (External Taper): Male pipe thread cut on a $1:16$ taper with $55^\circ$ Whitworth geometry.
- $Rc$ (Internal Taper): Female pipe thread cut on a matching $1:16$ taper for heavy industrial high-pressure pipework.
- $Rp$ (Internal Parallel): Female cylindrical thread (often called BSPP with thread-sealing capability) that allows a tapered $R$ male fitting to screw in until flank interference creates a seal.
BSPT vs NPT Distinction
While both BSPT and NPT share an identical $1:16$ taper rate ($0.75\text{ in/ft}$), they differ in two critical dimensions:
- Flank Angle: BSPT is $55^\circ$ with radiused roots/crests; NPT is $60^\circ$ with flattened (truncated) roots/crests.
- Pitch (TPI): $1/2\text{"}$ and $3/4\text{"}$ sizes happen to share $14\text{ TPI}$, but $1/8\text{"}$ ($28\text{ vs } 27\text{ TPI}$), $1/4\text{"}$ ($19\text{ vs } 18\text{ TPI}$), and $1\text{"}$ ($11\text{ vs } 11.5\text{ TPI}$) have incompatible pitches and will strip if forced.