NPT male thread envelopes 1/8 to 1 in (ASME B1.20.1) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/npt-thread-envelopes/  (CC BY 4.0)
Basis: ANSI/ASME B1.20.1 NPT major diameters, TPI, tap drills and effective thread length L2
Catalog version: 2026-10-20

Sizes: see table below
- 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.

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

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, 6 parts): https://xemstudio.com/files/hardware/npt-thread-envelopes/index.json
API: https://xemstudio.com/api/geometry?object=npt-thread-envelopes