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Pipe thread envelopes: NPT (ANSI B1.20.1) and BSP (ISO 228 / 7), 1/8 to 1 inch
What is the outside diameter of a 1/4 NPT thread and the drill size for it? · Hardware · 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
Long Earth Society build part. Pressure-tap and thermocouple ports on the LES engine heads. The engines · all LES parts
- sizes
- 6 in table
Notes
- 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.
Sizes
| 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 |
Where the files are
Other publishers' files are linked, not hosted. "verify" = check the license at the link before commercial use.
| Wikipedia | National pipe thread | web | CC BY-SA 4.0 (text) |
| Wikipedia | British Standard Pipe | web | CC BY-SA 4.0 (text) |
| Long Earth Society | Engine builds that use these parts (videos and write-ups) | web, video | LES site |
Free files
★ Free files 12 parts. Generated by Xem Studio from the standard; threads are plain cylinders at major diameter. CC BY 4.0. index.json
| npt-1-8-male | 1/8 NPT male, major OD 10.29 mm, 27 TPI, 8.6 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| npt-1-4-male | 1/4 NPT male, major OD 13.72 mm, 18 TPI, 11.6 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| npt-3-8-male | 3/8 NPT male, major OD 17.15 mm, 18 TPI, 11.6 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| npt-1-2-male | 1/2 NPT male, major OD 21.34 mm, 14 TPI, 15.0 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| npt-3-4-male | 3/4 NPT male, major OD 26.67 mm, 14 TPI, 15.0 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| npt-1-male | 1 NPT male, major OD 33.4 mm, 11.5 TPI, 18.0 mm engagement; plain cylinder at OD (taper not modeled) | STEP · STL |
| bsp-1-8-male | G1/8 BSPP male, major OD 9.73 mm, 28 TPI, 14 mm; plain cylinder at OD | STEP · STL |
| bsp-1-4-male | G1/4 BSPP male, major OD 13.16 mm, 19 TPI, 14 mm; plain cylinder at OD | STEP · STL |
| bsp-3-8-male | G3/8 BSPP male, major OD 16.66 mm, 19 TPI, 14 mm; plain cylinder at OD | STEP · STL |
| bsp-1-2-male | G1/2 BSPP male, major OD 20.96 mm, 14 TPI, 14 mm; plain cylinder at OD | STEP · STL |
| bsp-3-4-male | G3/4 BSPP male, major OD 26.44 mm, 14 TPI, 14 mm; plain cylinder at OD | STEP · STL |
| bsp-1-male | G1 BSPP male, major OD 33.25 mm, 11 TPI, 14 mm; plain cylinder at OD | STEP · STL |
Plain text: sheet.txt · JSON: /api/geometry?object=npt-bsp-thread-envelopes
Fits with
Questions
What is the OD of 1/4 NPT?
13.72 mm (0.540 in) at the large end, 18 TPI; tap drill 11.6 mm (7/16 in). 1/8 NPT is 10.29 mm, 27 TPI, 8.6 mm drill.
Are NPT and BSP interchangeable?
No. Same nominal names, but NPT is 60° and BSP 55°, the ODs differ (1/2 NPT 21.34 vs G1/2 20.96 mm) and 1/8, 1/4 and 3/8 have different TPI. They will cross-thread and leak.
How deep does an NPT thread engage?
Hand-tight about 4 mm for 1/8, 6 mm for 1/4 and 3/8, 8 mm for 1/2 and 3/4; add 1.5–2 turns with a wrench.