Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
The 1/4 BSP thread has a major outside diameter of 13.157 mm with 19 threads per inch (1.337 mm pitch). The number in the name is NOT the thread diameter: 1/4 BSP refers to the historical pipe bore, which is why a 1/4 BSP male measures 13.157 mm across, not 1/4 inch. Both the parallel form (G1/4, BS EN ISO 228-1) and the taper form (R1/4, BS EN 10226-1) share these thread dimensions - the difference is the 1:16 taper on R threads.
1/4 BSP dimensions
| Property | Value | Standard |
|---|
| Parallel designation | G1/4 (BSPP) | BS EN ISO 228-1 |
| Taper designation | R1/4 external / Rp1/4 internal parallel | BS EN 10226-1 |
| Threads per inch (TPI) | 19 | shared |
| Pitch | 1.337 mm | computed (25.4/TPI) |
| Major diameter | 13.157 mm | ISO 228-1 |
| Pitch diameter | 12.301 mm | computed (Whitworth form) |
| Minor diameter | 11.445 mm | computed (Whitworth form) |
| Tapping drill (G, parallel) | 11.44 mm (= minor dia; round up to stock drill) | derived |
| Thread form | Whitworth, 55 degrees, rounded crests and roots | shared |
G1/4, R1/4, Rp1/4: which one seals, and how
G1/4 (BSPP, parallel) does not seal on the threads: it needs a bonded washer, O-ring or face seal, and it torques to a positive stop. R1/4 (BSPT, taper external) seals on the thread flanks with PTFE tape or sealant as it wedges in. Rp1/4 is a PARALLEL female made to take a taper male - the common combination on valves and fittings. Never rely on a G male into an Rp female to seal without a washer: parallel-into-parallel has no wedge action.
How to identify a 1/4 BSP thread
Measure the male thread's outside diameter with calipers and count TPI against a thread gauge or a steel rule. 13.157 mm and 19 TPI together confirm 1/4 BSP. If the diameter is close but the count is 60-degree NPT-style, it is an American pipe thread - BSP and NPT do not interchange: the flank angles differ (55 vs 60 degrees) and mixing them produces a joint that feels tight and leaks under pressure.
Where 1/4 BSP turns up
Pneumatic fittings, gauges, small hydraulic ports and instrumentation.