This record deliberately preserves the disagreement instead of selecting a single false consensus value. NPT is tapered, and drill preparation, reaming practice, tap design and gauging context matter.
Why three values appear
The Cle-Line catalog distinguishes whether a taper reamer is used before tapping. The Travers chart presents an R drill recommendation. These are manufacturer or distributor recommendations, not values merged into an invented average.
Select by process
Identify the exact NPT tap, work material and whether the documented process includes taper reaming. Then use that tool maker’s current chart and preserve its context on the setup sheet.
Verify the result
Drill diameter alone does not establish a correct tapered pipe thread. Control tapping depth and inspect with the gauge and procedure required by the drawing or governing specification.
Do not apply a straight-thread major-diameter-minus-pitch rule to NPT, and do not treat hand fit as proof of sealing performance.
Technical references
- Cle-Line C01112 Tap and Drill Recommendations — Greenfield Industries / Cle-Line. Accessed 2026-11-09.
- Decimal Equivalent & Drill Size Chart — Travers Tool Co.. Accessed 2026-11-09.
- ASME B1.20.1-2013 (R2018) Pipe Threads, General Purpose, Inch — ASME. Accessed 2026-11-10.
Standards references identify scope and terminology; protected dimensional tables are not reproduced.