The 2 BSP thread has a major outside diameter of 59.614 mm with 11 threads per inch (2.309 mm pitch). The number in the name is NOT the thread diameter: 2 BSP refers to the historical pipe bore, which is why a 2 BSP male measures 59.614 mm across, not 2 inch. Both the parallel form (G2, BS EN ISO 228-1) and the taper form (R2, BS EN 10226-1) share these thread dimensions - the difference is the 1:16 taper on R threads.
2 BSP dimensions
| Property | Value | Standard |
|---|
| Parallel designation | G2 (BSPP) | BS EN ISO 228-1 |
| Taper designation | R2 external / Rp2 internal parallel | BS EN 10226-1 |
| Threads per inch (TPI) | 11 | shared |
| Pitch | 2.309 mm | computed (25.4/TPI) |
| Major diameter | 59.614 mm | ISO 228-1 |
| Pitch diameter | 58.135 mm | computed (Whitworth form) |
| Minor diameter | 56.657 mm | computed (Whitworth form) |
| Tapping drill (G, parallel) | 56.66 mm (= minor dia; round up to stock drill) | derived |
| Thread form | Whitworth, 55 degrees, rounded crests and roots | shared |
G2, R2, Rp2: which one seals, and how
G2 (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. R2 (BSPT, taper external) seals on the thread flanks with PTFE tape or sealant as it wedges in. Rp2 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 2 BSP thread
Measure the male thread's outside diameter with calipers and count TPI against a thread gauge or a steel rule. 59.614 mm and 11 TPI together confirm 2 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 2 BSP turns up
Larger pipework, pump flanged-to-screwed adaptors, industrial valves and plant installations.
Frequently Asked Questions
What diameter is a 2 BSP thread?
A 2 BSP male thread measures 59.614 mm across the outside diameter, with 11 threads per inch. The 2 in the name refers to the historical pipe bore, not the thread size.
What is the difference between G2 and R2?
G2 is the parallel (BSPP) version and seals with a washer or O-ring on a face, not on the threads. R2 is the taper (BSPT) version with a 1:16 taper that seals on the thread flanks with PTFE tape or sealant. Thread diameter and pitch are the same on both.
What is the tapping drill for G2?
The minor diameter of 2 BSP is 56.66 mm, so use the nearest stock drill at or just above that figure. Drilling undersize makes tapping a 11 TPI Whitworth-form thread hard work; oversize reduces thread engagement.
Will 2 BSP fit 2 NPT?
No. BSP uses a 55-degree Whitworth thread form and NPT uses 60 degrees, and the pitches differ on most sizes. The two may start to engage but will not seal reliably - a BSP-to-NPT adaptor is the correct fix.