Verified by Rachel Mayfield, Supply Chain Analyst - August 2026
The 5/8 BSP thread has a major outside diameter of 22.911 mm with 14 threads per inch (1.814 mm pitch). The number in the name is NOT the thread diameter: 5/8 BSP refers to the historical pipe bore, which is why a 5/8 BSP male measures 22.911 mm across, not 5/8 inch. Both the parallel form (G5/8, BS EN ISO 228-1) and the taper form (R5/8, BS EN 10226-1) share these thread dimensions - the difference is the 1:16 taper on R threads.
5/8 BSP dimensions
| Property | Value | Standard |
|---|
| Parallel designation | G5/8 (BSPP) | BS EN ISO 228-1 |
| Taper designation | R5/8 external / Rp5/8 internal parallel | BS EN 10226-1 |
| Threads per inch (TPI) | 14 | shared |
| Pitch | 1.814 mm | computed (25.4/TPI) |
| Major diameter | 22.911 mm | ISO 228-1 |
| Pitch diameter | 21.749 mm | computed (Whitworth form) |
| Minor diameter | 20.588 mm | computed (Whitworth form) |
| Tapping drill (G, parallel) | 20.59 mm (= minor dia; round up to stock drill) | derived |
| Thread form | Whitworth, 55 degrees, rounded crests and roots | shared |
G5/8, R5/8, Rp5/8: which one seals, and how
G5/8 (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. R5/8 (BSPT, taper external) seals on the thread flanks with PTFE tape or sealant as it wedges in. Rp5/8 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 5/8 BSP thread
Measure the male thread's outside diameter with calipers and count TPI against a thread gauge or a steel rule. 22.911 mm and 14 TPI together confirm 5/8 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 5/8 BSP turns up
Plumbing, hydraulic and pneumatic fittings, pump connections, valves and general plant pipework across UK and European equipment.