# 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?

- Category: Hardware › Stirling & thermoacoustic
- Kind: 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
- Page: https://xemstudio.com/objects/npt-bsp-thread-envelopes/ · JSON: https://xemstudio.com/api/geometry?object=npt-bsp-thread-envelopes · Plain text: https://xemstudio.com/objects/npt-bsp-thread-envelopes/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## 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

- Wikipedia: [National pipe thread](https://en.wikipedia.org/wiki/National_pipe_thread) — web — CC BY-SA 4.0 (text)
- Wikipedia: [British Standard Pipe](https://en.wikipedia.org/wiki/British_Standard_Pipe) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds that use these parts (videos and write-ups)](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (12 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/index.json

- npt-1-8-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-8-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-8-male.stl) — 1/8 NPT male, major OD 10.29 mm, 27 TPI, 8.6 mm engagement; plain cylinder at OD (taper not modeled)
- npt-1-4-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-4-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-4-male.stl) — 1/4 NPT male, major OD 13.72 mm, 18 TPI, 11.6 mm engagement; plain cylinder at OD (taper not modeled)
- npt-3-8-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-8-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-8-male.stl) — 3/8 NPT male, major OD 17.15 mm, 18 TPI, 11.6 mm engagement; plain cylinder at OD (taper not modeled)
- npt-1-2-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-2-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-2-male.stl) — 1/2 NPT male, major OD 21.34 mm, 14 TPI, 15.0 mm engagement; plain cylinder at OD (taper not modeled)
- npt-3-4-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-4-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-3-4-male.stl) — 3/4 NPT male, major OD 26.67 mm, 14 TPI, 15.0 mm engagement; plain cylinder at OD (taper not modeled)
- npt-1-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/npt-1-male.stl) — 1 NPT male, major OD 33.4 mm, 11.5 TPI, 18.0 mm engagement; plain cylinder at OD (taper not modeled)
- bsp-1-8-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-8-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-8-male.stl) — G1/8 BSPP male, major OD 9.73 mm, 28 TPI, 14 mm; plain cylinder at OD
- bsp-1-4-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-4-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-4-male.stl) — G1/4 BSPP male, major OD 13.16 mm, 19 TPI, 14 mm; plain cylinder at OD
- bsp-3-8-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-8-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-8-male.stl) — G3/8 BSPP male, major OD 16.66 mm, 19 TPI, 14 mm; plain cylinder at OD
- bsp-1-2-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-2-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-2-male.stl) — G1/2 BSPP male, major OD 20.96 mm, 14 TPI, 14 mm; plain cylinder at OD
- bsp-3-4-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-4-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-3-4-male.stl) — G3/4 BSPP male, major OD 26.44 mm, 14 TPI, 14 mm; plain cylinder at OD
- bsp-1-male: [STEP](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-male.step) · [STL](https://xemstudio.com/files/hardware/npt-bsp-thread-envelopes/bsp-1-male.stl) — G1 BSPP male, major OD 33.25 mm, 11 TPI, 14 mm; plain cylinder at OD

## 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.

## Fits with

- [Schedule 40 pipe and PVC fittings, 1/2 to 2 inch](https://xemstudio.com/objects/schedule-40-pipe/)
- [Copper pipe (ASTM B88 Type L and M), 1/4 to 2 inch](https://xemstudio.com/objects/copper-pipe-type-l-m/)
- [O-rings, common metric sizes (ISO 3601)](https://xemstudio.com/objects/o-rings-metric/)
- [Tri-clamp end cap and gasket, 1.5 and 2 inch](https://xemstudio.com/objects/tri-clamp-end-cap/)