# NPT male thread envelopes 1/8 to 1 in (ASME B1.20.1)

> 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, TPI, tap drills and effective thread length L2
- Page: https://xemstudio.com/objects/npt-thread-envelopes/ · JSON: https://xemstudio.com/api/geometry?object=npt-thread-envelopes · Plain text: https://xemstudio.com/objects/npt-thread-envelopes/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

**Exact dimensions.** True size, with no printing tolerance added.

## Notes

- Each file is a plain cylinder at the NPT major OD, the length of the effective thread (L2), with a 0.8 mm chamfer on the end; the 1:16 taper and the thread form are not modelled.
- 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 pipe ODs). L2: 6.7, 10.2, 10.4, 13.6, 13.9, 17.3 mm.
- Tap drills: 1/8 8.6 (R), 1/4 11.1 (7/16), 3/8 14.7 (37/64), 1/2 18.3 (23/32), 3/4 23.4 (59/64), 1 in 29.4 mm (1-5/32).
- Hand-tight engagement is about 4 mm on 1/8, 6 mm on 1/4 and 3/8, 8 mm on 1/2 and 3/4; NPT is a 60° thread and does not interchange with BSP.
- On my engine heads 1/8 and 1/4 NPT are the pressure-tap and thermocouple ports.

## Sizes

| size | od_mm | tpi | tap_drill_mm | l2_mm |
|---|---|---|---|---|
| 1/8 | 10.29 | 27 | 8.6 | 6.7 |
| 1/4 | 13.72 | 18 | 11.1 | 10.2 |
| 3/8 | 17.15 | 18 | 14.7 | 10.4 |
| 1/2 | 21.34 | 14 | 18.3 | 13.6 |
| 3/4 | 26.67 | 14 | 23.4 | 13.9 |
| 1 | 33.4 | 11.5 | 29.4 | 17.3 |

## 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 (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/npt-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, 6.7 mm effective thread length (L2); 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, 10.2 mm effective thread length (L2); 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, 10.4 mm effective thread length (L2); 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, 13.6 mm effective thread length (L2); 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, 13.9 mm effective thread length (L2); 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, 17.3 mm effective thread length (L2); plain cylinder at OD (taper not modeled)

## How it fits

### Goes in or on
- [Tri-clamp end cap, 6 mm flat: 1.5 and 2 in](https://xemstudio.com/objects/tri-clamp-end-cap/): 1/8 or 1/4 NPT port in an engine head

### Connects to
- [Schedule 40 pipe and PVC fittings, 1/2 to 2 inch](https://xemstudio.com/objects/schedule-40-pipe/): NPT major OD = the pipe OD

### Alternatives
- [BSP (G) male thread envelopes 1/8 to 1 in (ISO 228-1)](https://xemstudio.com/objects/bsp-thread-envelopes/): BSP is 55° with different ODs; not interchangeable

## Questions

**What is the OD of 1/4 NPT?**

13.72 mm (0.540 in) at the large end, 18 TPI; tap drill 11.1 mm (7/16 in).

**What is the tap drill for 1/8 NPT?**

8.6 mm (letter R); the thread is 10.29 mm OD, 27 TPI.

**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.

## Fits with

- [BSP (G) male thread envelopes 1/8 to 1 in (ISO 228-1)](https://xemstudio.com/objects/bsp-thread-envelopes/)
- [Schedule 40 pipe and PVC fittings, 1/2 to 2 inch](https://xemstudio.com/objects/schedule-40-pipe/)
- [Tri-clamp end cap, 6 mm flat: 1.5 and 2 in](https://xemstudio.com/objects/tri-clamp-end-cap/)
- [O-rings, common metric sizes (ISO 3601)](https://xemstudio.com/objects/o-rings-metric/)