MechPilotTap drill sizes for ISO metric coarse threads (ISO 2306)
The drill to use before tapping an ISO metric coarse thread, with the thread pitch.
| Thread | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M1.6 | 0.35 | 1.25 |
| M1.8 | 0.35 | 1.45 |
| M2 | 0.4 | 1.6 |
| M2.2 | 0.45 | 1.75 |
| M2.5 | 0.45 | 2.05 |
| M3 | 0.5 | 2.5 |
| M3.5 | 0.6 | 2.9 |
| M4 | 0.7 | 3.3 |
| M4.5 | 0.75 | 3.7 |
| M5 | 0.8 | 4.2 |
| M6 | 1 | 5 |
| M7 | 1 | 6 |
| M8 | 1.25 | 6.8 |
| M9 | 1.25 | 7.8 |
| M10 | 1.5 | 8.5 |
| M11 | 1.5 | 9.5 |
| M12 | 1.75 | 10.2 |
| M14 | 2 | 12 |
| M16 | 2 | 14 |
| M18 | 2.5 | 15.5 |
| M20 | 2.5 | 17.5 |
| M22 | 2.5 | 19.5 |
| M24 | 3 | 21 |
| M27 | 3 | 24 |
| M30 | 3.5 | 26.5 |
| M33 | 3.5 | 29.5 |
| M36 | 4 | 32 |
| M39 | 4 | 35 |
| M42 | 4.5 | 37.5 |
| M45 | 4.5 | 40.5 |
| M48 | 5 | 43 |
| M52 | 5 | 47 |
| M56 | 5.5 | 50.5 |
ISO 261:1998, Table 2, column 'coarse'; same values in ISO 2306:1972, Table 1. ISO 2306:1972, Table 1, column 'Drill diameter'.
drill diameter ≈ nominal diameter − pitch — ISO 2306:1972, clause 1: the sizes of drills for parallel screw threads of normal length of engagement, the drill diameter being approximately equal to the nominal diameter of the thread minus the pitch
Notes from the data file
- ISO 2306:1972, clause 0: the hole a drill makes depends on the drill point, the material, the lubricant and the machining conditions; in soft materials the tapped minor diameter can end up smaller than the drill. The table is a guide for conventional (cutting) tapping; it does not apply to thread forming.
- ISO 2306:1972, clause 1: the drill diameters, except four sizes the standard marks with an asterisk, are selected from the ISO/R 235 twist drill sizes; stocked drill diameters should be used whenever possible (clause 0).
Source and verification
- Standard
- ISO 2306:1972 Drills for use prior to tapping screw threads, clause 3.1 and Table 1 (coarse pitch series); pitches cross-checked with ISO 261:1998 ISO general purpose metric screw threads - General plan, Table 2 (coarse column)
- Verification
primary(the standard’s own text was seen)- Verified from
- https://law.resource.org/pub/in/bis/S01/is.10952.1984.pdf (IS 10952:1984, identical with ISO 2306:1972: clauses 0, 1 and 3.1, Table 1; read from the rendered page on 2026-10-22)
- https://law.resource.org/pub/in/bis/S01/is.4218.2.2001.pdf (IS 4218 (Part 2):2001, identical with ISO 261:1998: clause 5, Table 2; read from the rendered pages on 2026-10-22)
- Verification notes
- pitch and drill were read from ISO 2306:1972 Table 1 (nominal diameter, pitch, minor diameter limits for 5H/6H/7H, drill diameter). Every pitch agrees with the coarse column of ISO 261:1998 Table 2.
- ISO 2306:1972 Table 1 ends at M56. M60 (pitch 5.5) and M64 (pitch 6) have coarse pitches in ISO 261:1998 Table 2 but no drill in ISO 2306; they are left out (see docs/to-verify.md). Use `rule` for them only with the user's confirmation.
- ISO 262 (selected sizes for bolts, screws and nuts) was not read; the sizes here are the ones ISO 2306 lists, which include second- and third-choice diameters (M1.8, M2.2, M3.5, M4.5, M7, M9, M11, ...) of ISO 261.
- Data file
library/holes/tap-drills.json
Values that could not be checked against the standard are left out and listed in docs/to-verify.md. Check the standard itself before you rely on a value for production, and report any mistake on GitHub. to-verify.md · Issues