Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
The 1-1/4 BSP thread has a major outside diameter of 41.910 mm with 11 threads per inch (2.309 mm pitch). The number in the name is NOT the thread diameter: 1-1/4 BSP refers to the historical pipe bore, which is why a 1-1/4 BSP male measures 41.910 mm across, not 1-1/4 inch. Both the parallel form (G1-1/4, BS EN ISO 228-1) and the taper form (R1-1/4, BS EN 10226-1) share these thread dimensions - the difference is the 1:16 taper on R threads.
1-1/4 BSP dimensions
| Property | Value | Standard |
|---|
| Parallel designation | G1-1/4 (BSPP) | BS EN ISO 228-1 |
| Taper designation | R1-1/4 external / Rp1-1/4 internal parallel | BS EN 10226-1 |
| Threads per inch (TPI) | 11 | shared |
| Pitch | 2.309 mm | computed (25.4/TPI) |
| Major diameter | 41.910 mm | ISO 228-1 |
| Pitch diameter | 40.431 mm | computed (Whitworth form) |
| Minor diameter | 38.953 mm | computed (Whitworth form) |
| Tapping drill (G, parallel) | 38.95 mm (= minor dia; round up to stock drill) | derived |
| Thread form | Whitworth, 55 degrees, rounded crests and roots | shared |
G1-1/4, R1-1/4, Rp1-1/4: which one seals, and how
G1-1/4 (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. R1-1/4 (BSPT, taper external) seals on the thread flanks with PTFE tape or sealant as it wedges in. Rp1-1/4 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 1-1/4 BSP thread
Measure the male thread's outside diameter with calipers and count TPI against a thread gauge or a steel rule. 41.910 mm and 11 TPI together confirm 1-1/4 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 1-1/4 BSP turns up
Larger pipework, pump flanged-to-screwed adaptors, industrial valves and plant installations.