BSP BSW British threads
The British BSP (British Standard Pipe) and BSW (British Standard Whitworth) threads constitute the historical thread system with a 55° profile defined by Joseph Whitworth in 1841. BSP designates pipe threads (cylindrical and tapered) standardized in ISO 228 and ISO 7, while BSW covers coarse pitch fastening threads, complemented by the BSF (British Standard Fine) series. Although partially replaced by metric standards, they remain in use in hydraulic and pneumatic installations, and machinery restoration.
Classification
Section titled “Classification”The main families of British threads covered in this reference are:
BSPP (G) – Parallel pipe thread
Cylindrical thread with no taper, seals via O-ring or conical seat, not by the thread. Complies with ISO 228.
BSPT (R/Rp) – Tapered pipe thread
Thread with 1:16 taper that seals on the thread. Male is designated R and female Rp. Complies with ISO 7.
BSW – Whitworth coarse pitch
Fastening thread with 55° angle, used in general hardware and old machinery. Standardized according to BS 84.
BSF – Whitworth fine pitch
Fine pitch variant of BSW for applications requiring greater resistance to vibration; not covered in detail in this datasheet.
Dimensions of BSP pipe threads
Section titled “Dimensions of BSP pipe threads”Parallel BSPP (G) thread
Section titled “Parallel BSPP (G) thread”Designation, main diameters, number of threads per inch and pitch.
| Nominal size | Major diameter | Minor diameter | Threads per inch and pitch |
|---|---|---|---|
| G 1/16 | 7,723 mm / 0,3041 in | 6,561 mm / 0,2583 in | 0,907 mm / 28 threads/in |
| G 1/8 | 9,728 mm / 0,3830 in | 8,566 mm / 0,3372 in | 0,907 mm / 28 threads/in |
| G 1/4 | 13,157 mm / 0,5180 in | 11,445 mm / 0,4506 in | 1,337 mm / 19 threads/in |
| G 3/8 | 16,662 mm / 0,6560 in | 14,950 mm / 0,5886 in | 1,337 mm / 19 threads/in |
| G 1/2 | 20,955 mm / 0,8250 in | 18,631 mm / 0,7335 in | 1,814 mm / 14 threads/in |
| G 3/4 | 26,441 mm / 1,0410 in | 24,117 mm / 0,9495 in | 1,814 mm / 14 threads/in |
| G 1 | 33,249 mm / 1,3090 in | 30,291 mm / 1,1925 in | 2,309 mm / 11 threads/in |
| G 2 | 59,614 mm / 2,3470 in | 56,656 mm / 2,2305 in | 2,309 mm / 11 threads/in |
Tapered BSPT (R/Rp) thread
Section titled “Tapered BSPT (R/Rp) thread”The table shows the major diameter of the male and the minor diameter of the female; the 1:16 taper allows sealing on the thread.
| Nominal size | Major diameter male (R) | Minor diameter female (Rp) | Threads per inch and pitch |
|---|---|---|---|
| R 1/16 · Rp 1/16 | 7,723 mm / 0,3041 in | 6,490 mm / 0,2555 in | 0,907 mm / 28 threads/in |
| R 1/8 · Rp 1/8 | 9,728 mm / 0,3830 in | 8,495 mm / 0,3344 in | 0,907 mm / 28 threads/in |
| R 1/4 · Rp 1/4 | 13,157 mm / 0,5180 in | 11,341 mm / 0,4465 in | 1,337 mm / 19 threads/in |
| R 3/8 · Rp 3/8 | 16,662 mm / 0,6560 in | 14,846 mm / 0,5845 in | 1,337 mm / 19 threads/in |
| R 1/2 · Rp 1/2 | 20,955 mm / 0,8250 in | 18,489 mm / 0,7279 in | 1,814 mm / 14 threads/in |
| R 3/4 · Rp 3/4 | 26,441 mm / 1,0410 in | 23,975 mm / 0,9439 in | 1,814 mm / 14 threads/in |
| R 1 · Rp 1 | 33,249 mm / 1,3090 in | 30,111 mm / 1,1855 in | 2,309 mm / 11 threads/in |
| R 2 · Rp 2 | 59,614 mm / 2,3470 in | 56,476 mm / 2,2235 in | 2,309 mm / 11 threads/in |
Dimensions of BSW threads (Whitworth coarse pitch)
Section titled “Dimensions of BSW threads (Whitworth coarse pitch)”Main diameters and pitches of BSW bolts. The minor diameter corresponds to the male core, relevant for strength calculation.
| Nominal size (in) | Major diameter | Threads per inch and pitch | Minor diameter |
|---|---|---|---|
| 1/8 | 3,175 mm / 0,1250 in | 0,635 mm / 40 threads/in | 2,362 mm / 0,0930 in |
| 5/32 | 3,969 mm / 0,1563 in | 0,794 mm / 32 threads/in | 2,953 mm / 0,1163 in |
| 3/16 | 4,763 mm / 0,1875 in | 1,058 mm / 24 threads/in | 3,405 mm / 0,1341 in |
| 1/4 | 6,350 mm / 0,2500 in | 1,270 mm / 20 threads/in | 4,724 mm / 0,1860 in |
| 5/16 | 7,938 mm / 0,3125 in | 1,411 mm / 18 threads/in | 6,115 mm / 0,2407 in |
| 3/8 | 9,525 mm / 0,3750 in | 1,588 mm / 16 threads/in | 7,492 mm / 0,2950 in |
| 7/16 | 11,113 mm / 0,4375 in | 1,814 mm / 14 threads/in | 8,789 mm / 0,3460 in |
| 1/2 | 12,700 mm / 0,5000 in | 2,117 mm / 12 threads/in | 9,986 mm / 0,3931 in |
| 9/16 | 14,288 mm / 0,5625 in | 2,117 mm / 12 threads/in | 11,574 mm / 0,4557 in |
| 5/8 | 15,875 mm / 0,6250 in | 2,309 mm / 11 threads/in | 12,913 mm / 0,5084 in |
| 3/4 | 19,050 mm / 0,7500 in | 2,540 mm / 10 threads/in | 15,798 mm / 0,6220 in |
| 7/8 | 22,225 mm / 0,8750 in | 2,822 mm / 9 threads/in | 18,613 mm / 0,7328 in |
| 1 | 25,400 mm / 1,0000 in | 3,175 mm / 8 threads/in | 21,338 mm / 0,8401 in |
Recommended tightening torques for BSW fasteners
Section titled “Recommended tightening torques for BSW fasteners”Reference values for hexagonal head BSW bolts in grade 8.8 (medium carbon steel, quenched and tempered), lightly lubricated (coefficient of friction µ ≈ 0.15). Not applicable to joints with gasket or pipe threads.
| Nominal size (in) | Tightening torque |
|---|---|
| 1/4-20 | 10,8 N·m / 8,0 ft·lb |
| 5/16-18 | 23,0 N·m / 17,0 ft·lb |
| 3/8-16 | 40,7 N·m / 30,0 ft·lb |
| 7/16-14 | 54,2 N·m / 40,0 ft·lb |
| 1/2-12 | 81,3 N·m / 60,0 ft·lb |
| 9/16-12 | 108 N·m / 80,0 ft·lb |
| 5/8-11 | 162 N·m / 120 ft·lb |
| 3/4-10 | 271 N·m / 200 ft·lb |
| 7/8-9 | 366 N·m / 270 ft·lb |
| 1-8 | 542 N·m / 400 ft·lb |
Drill bits for threading BSPP (G) and BSPT (R/Rp)
Section titled “Drill bits for threading BSPP (G) and BSPT (R/Rp)”Recommended drill diameter for machining the hole prior to tapping. For parallel G thread it coincides with the minor diameter of the tap; for tapered Rp thread the core drill diameter of the tapered female is indicated.
| Nominal size | Drill for G thread (mm / in) | Drill for Rp thread (mm / in) |
|---|---|---|
| 1/8 | 8,6 mm / 0,339 in | 8,5 mm / 0,335 in |
| 1/4 | 11,5 mm / 0,453 in | 11,3 mm / 0,445 in |
| 3/8 | 15,0 mm / 0,591 in | 14,8 mm / 0,583 in |
| 1/2 | 18,6 mm / 0,732 in | 18,5 mm / 0,728 in |
| 3/4 | 24,1 mm / 0,949 in | 24,0 mm / 0,945 in |
| 1 | 30,3 mm / 1,193 in | 30,1 mm / 1,185 in |
| 2 | 56,7 mm / 2,232 in | 56,5 mm / 2,224 in |
The values for Rp correspond to the minor diameter of the female, valid for tapping with a tapered tap. In practice, it is common to start with a slightly smaller diameter and calibrate with the tap.
Allowable loads for BSW bolts grade 8.8
Section titled “Allowable loads for BSW bolts grade 8.8”Allowable axial tensile load based on a proof stress of 580 MPa (85% of the nominal yield strength of 640 MPa). The Whitworth thread stress area according to BS 84 is considered.
| Nominal size (in) | Stress area | Allowable load |
|---|---|---|
| 1/4-20 | 20,0 mm² / 0,0310 in² | 11,6 kN / 2,61 kip |
| 5/16-18 | 33,0 mm² / 0,0512 in² | 19,1 kN / 4,29 kip |
| 3/8-16 | 50,0 mm² / 0,0775 in² | 29,0 kN / 6,52 kip |
| 7/16-14 | 72,0 mm² / 0,112 in² | 41,8 kN / 9,40 kip |
| 1/2-12 | 99,0 mm² / 0,153 in² | 57,4 kN / 12,9 kip |
| 9/16-12 | 127 mm² / 0,197 in² | 73,7 kN / 16,6 kip |
| 5/8-11 | 159 mm² / 0,247 in² | 92,2 kN / 20,7 kip |
| 3/4-10 | 234 mm² / 0,363 in² | 136 kN / 30,6 kip |
| 7/8-9 | 326 mm² / 0,506 in² | 189 kN / 42,5 kip |
| 1-8 | 432 mm² / 0,670 in² | 251 kN / 56,4 kip |
Standard equivalences
Section titled “Standard equivalences”BSP and BSW threads are not interchangeable with the American NPT and UNC series, despite coincidences in diameters and pitches. The following table highlights the main geometric differences.
| BSP/BSW size | Nominal diameter (mm / in) | Closest American standard | American nominal diameter (mm / in) | Pitch (mm / TPI) | Interchangeable |
|---|---|---|---|---|---|
| G 1/8 (BSPP) | 9,728 / 0,383 | NPT 1/8 | 10,287 / 0,405 | 0,907 / 28 vs 0,941 / 27 | No – BSP thread 55° flat vs. NPT 60° tapered |
| G 1/2 (BSPP) | 20,955 / 0,825 | NPT 1/2 | 21,336 / 0,840 | 1,814 / 14 vs. 1,814 / 14 | No – different profile and major diameter |
| BSW 1/4-20 | 6,350 / 0,250 | UNC 1/4-20 | 6,350 / 0,250 | 1,270 / 20 vs. 1,270 / 20 | No – thread angle 55° (BSW) vs. 60° (UNC) |
| BSW 1/2-12 | 12,700 / 0,500 | UNC 1/2-13 | 12,700 / 0,500 | 2,117 / 12 vs. 1,954 / 13 | No – different pitch and angle |
These differences prevent assembly without damaging the threads. Specific adapters are required in conversions.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the pitch of a 1/2 inch BSP thread?
Section titled “What is the pitch of a 1/2 inch BSP thread?”The 1/2“ BSP thread (G1/2) has 14 threads per inch, which is equivalent to a pitch of 1,814 mm. This pitch is common to both the parallel and tapered versions of that size.
Can I connect a male BSPT thread with a female NPT thread?
Section titled “Can I connect a male BSPT thread with a female NPT thread?”No, the major diameter of an R 1/2 thread is 20,955 mm / 0,8250 in, while the NPT 1/2 measures 21,336 mm / 0,840 in, and the thread angle differs (55° vs. 60°). The connection would result in leakage or seizure.
What is the tensile strength of a BSW 1/4“ grade 8.8 bolt?
Section titled “What is the tensile strength of a BSW 1/4“ grade 8.8 bolt?”The allowable load for a BSW 1/4“ grade 8.8 bolt is approximately 11,6 kN / 2,61 kip, based on a stress area of 20,0 mm² and a proof stress of 580 MPa.
What drill size do I need to tap a G 3/8 hole?
Section titled “What drill size do I need to tap a G 3/8 hole?”For tapping G 3/8, a 15,0 mm (0,591 inch) drill is recommended, which corresponds to the minor diameter of the BSPP tap for that size, ensuring a correct internal thread profile.
What is the recommended tightening torque for a BSW 5/16“ bolt?
Section titled “What is the recommended tightening torque for a BSW 5/16“ bolt?”The recommended tightening torque for a BSW 5/16“ grade 8.8 lubricated bolt is 23,0 N·m / 17,0 ft·lb, sufficient to develop the design preload without exceeding the yield strength.
How does BSPP thread differ from BSPT?
Section titled “How does BSPP thread differ from BSPT?”The BSPT thread has a taper of 1 in 16 (approximately 3.58° per side) and seals on the thread; the major diameter of the R 1/2 male is 20,955 mm / 0,8250 in, identical to the BSPP nominal, but the Rp female has a minor diameter of 18,489 mm / 0,7279 in, smaller than in the parallel version, so that the interference creates the seal.