Pipe thread envelopes: NPT (ANSI B1.20.1) and BSP (ISO 228 / 7), 1/8 to 1 inch — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/npt-bsp-thread-envelopes/  (CC BY 4.0)
Basis: ANSI/ASME B1.20.1 NPT major diameters and tap drills; ISO 228-1 (G, parallel) and ISO 7-1 (R/Rc, taper) BSP major diameters
Catalog version: 2026-10-20

Sizes: see table below
- NPT major OD: 1/8 10.29, 1/4 13.72, 3/8 17.15, 1/2 21.34, 3/4 26.67, 1 in 33.40 mm (the Schedule pipe ODs). Taper 1:16; hand-tight engagement 1/8: 4 mm, 1/4 and 3/8: 6 mm, 1/2 and 3/4: 8 mm.
- NPT tap drills: 1/8 → 8.6 mm (11/32), 1/4 → 11.6 (7/16), 3/8 → 15.0 (37/64), 1/2 → 18.0 (23/32)... use the listed drill then the taper tap to 2–3 turns past the first full thread.
- BSP (G) major OD: 1/8 9.73, 1/4 13.16, 3/8 16.66, 1/2 20.96, 3/4 26.44, 1 in 33.25 mm, 55° thread, parallel (G) seals on a washer or O-ring, taper (R) on the thread. NPT and BSP do not interchange: 1/2 NPT is 14 TPI 60°, 1/2 BSP is 14 TPI 55° with a different OD.
- In a print: model a plain hole at the tap-drill size and tap it, or use a brass insert; printed pipe threads leak above about 2 bar.
- Why it's here: 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports on the LES engine heads; 1/2 BSP is the common gas-fitting thread.

size  npt_od_mm  npt_tpi  npt_tap_drill_mm  bsp_g_od_mm  bsp_tpi
1/8  10.29  27  8.6  9.73  28
1/4  13.72  18  11.6  13.16  19
3/8  17.15  18  11.6  16.66  19
1/2  21.34  14  15.0  20.96  14
3/4  26.67  14  15.0  26.44  14
1  33.4  11.5  18.0  33.25  11

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- Wikipedia: National pipe thread — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/National_pipe_thread
- Wikipedia: British Standard Pipe — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/British_Standard_Pipe
- Long Earth Society: Engine builds that use these parts (videos and write-ups) — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 12 parts): https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/index.json
API: https://xemstudio.com/api/geometry?object=npt-bsp-thread-envelopes