The 1/16 BSP thread has a major outside diameter of 7.723 mm with 28 threads per inch (0.907 mm pitch). It is the smallest size in the BSP series. The number in the name is not the thread diameter: 1/16 BSP refers to the historical pipe bore, which is why a 1/16 BSP male measures 7.723 mm across, not 1/16 inch. The parallel form (G1/16) and the taper form (R1/16) share these thread dimensions.
1/16 BSP dimensions
| Property |
1/16 BSP |
1/8 BSP (for comparison) |
Standard |
| Parallel designation | G1/16 (BSPP) | G1/8 (BSPP) | BS EN ISO 228-1 |
| Taper designation | R1/16 external / Rp1/16 internal parallel | R1/8 / Rp1/8 | BS EN 10226-1 |
| Threads per inch (TPI) | 28 | 28 | shared |
| Pitch | 0.907 mm | 0.907 mm | 25.4 / TPI |
| Major diameter | 7.723 mm | 9.728 mm | ISO 228-1 |
| Pitch diameter | 7.142 mm | 9.147 mm | ISO 228-1 |
| Minor diameter | 6.561 mm | 8.566 mm | ISO 228-1 |
| Thread depth | 0.581 mm | 0.581 mm | Whitworth form |
| Gauge length, taper thread | 4 mm | 4 mm | BS EN 10226-1 |
| Thread form | Whitworth, 55 degrees, rounded crests and roots | same | shared |
1/16 BSP and 1/8 BSP have the same 28 TPI pitch, so a thread gauge alone will not separate them. Measure the outside diameter: 7.723 mm is 1/16 and 9.728 mm is 1/8.
Tapping drill for 1/16 BSP
The minor diameter of the thread is 6.561 mm, and the tapping drill has to be at or just above that. Published tapping drill charts for this size do not agree, because they assume different amounts of thread engagement and different allowances for the taper form. Use the drill size recommended by the maker of your tap.
G1/16, R1/16, Rp1/16: which one seals, and how
G1/16 (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. R1/16 (BSPT, taper external) seals on the thread flanks with PTFE tape or sealant as it wedges in. Rp1/16 is a parallel female made to take a taper male. Never rely on a G male into an Rp female to seal without a washer, because parallel into parallel has no wedge action.
Where 1/16 BSP turns up
Small pneumatic and instrument connections, lubrication fittings, gauge ports and miniature valves.
References
- BS EN ISO 228-1 - Pipe threads where pressure-tight joints are not made on the threads (G)
- BS EN 10226-1 - Pipe threads where pressure-tight joints are made on the threads (R and Rp)