M9×1 Thread Dimensions and Tap Drill
M9×1 is an ISO metric fine-pitch thread: the 9 mm diameter with a 1 mm pitch (25.4 threads per inch) instead of the coarse 1.25 mm of M9×1.25. The thread is 0.14 mm shallower, so the tensile stress area rises from 48.1 to 51.0 mm² (6% more) and the lead angle falls from 2.78° to 2.18°.
- Tap drill8 mm77% thread
- Pitch1 mm25.4 TPI
- Major diameter9.000 mm0.3543 in
- Pitch diameter8.350 mm0.3288 in
- Minor diameter (nut)7.917 mm0.3117 in
- Tensile stress area51.0 mm²0.0791 in²
Thread profile
M9×1 basic dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 9.000 | 0.3543 |
| Pitch P | 1.000 | 0.0394 |
| Pitch diameter d2 = D2 | 8.350 | 0.3288 |
| Minor diameter, internal thread D1 | 7.917 | 0.3117 |
| Minor diameter, external thread d3 | 7.773 | 0.3060 |
| Height of fundamental triangle H | 0.866 | 0.0341 |
| Thread height, internal (5H/8) | 0.541 | 0.0213 |
| Thread height, external (17H/24) | 0.613 | 0.0242 |
| Crest flat of external thread (P/8) | 0.125 | 0.0049 |
| Crest flat of internal thread (P/4) | 0.250 | 0.0098 |
| Property | Value |
|---|---|
| Threads per inch | 25.40 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 2.18° |
| Tensile stress area As | 51.0 mm² (0.0791 in²) |
Basic (nominal) sizes per ISO 724, before tolerances. d3 is the root diameter of a bolt with the rounded root that ISO 898-1 uses for the stress area.
M9×1 tolerance limits (6g bolt, 6H nut)
6g and 6H are the general-purpose tolerance classes for commercial screws and nuts. The 6g screw has a fundamental deviation of 26 µm, so even at its largest it sits 0.026 mm below the basic size and leaves room for a thin plating.
| External thread 6g | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Major diameter d | 8.974 | 8.794 | 0.180 |
| Pitch diameter d2 | 8.324 | 8.212 | 0.112 |
| Internal thread 6H | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Minor diameter D1 | 8.153 | 7.917 | 0.236 |
| Pitch diameter D2 | 8.500 | 8.350 | 0.150 |
Computed from the ISO 965-1 deviations and tolerances for normal length of engagement; the results match the limits tabulated in ISO 965-2.
M9×1 tap drill size
Tap drill for M9×1: 8 mm
This leaves about 77% thread in the tapped hole.
The rule for metric tap drills is diameter minus pitch: 9 − 1 = 8 mm, which is a stocked drill size.
The 6H minor diameter of the finished nut thread must lie between 7.917 and 8.153 mm. The recommended drill falls inside that band.
Other drills that work for M9×1
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 6H limits | Thread engagement |
|---|---|---|---|---|---|
| 7.9 mm | 7.90 | 0.3110 | 85% | no | |
| 8 mmrecommended | 8.00 | 0.3150 | 77% | yes | |
| O (0.3160 in) | 8.03 | 0.3160 | 75% | yes | |
| 8.1 mm | 8.10 | 0.3189 | 69% | yes | |
| 8.2 mm | 8.20 | 0.3228 | 62% | no | |
| 8.25 mm | 8.25 | 0.3248 | 58% | no | |
| 8.3 mm | 8.30 | 0.3268 | 54% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for M9
ISO 273 does not list a clearance hole for M9; its table goes from M8 to M10. The neighbouring sizes leave 0.4 to 0.5 mm of clearance in the fine series and about 1 mm in the medium series.
M9×1 strength
ISO 898-1 tabulates fine-pitch loads only for a selected set of threads from M8×1 to M39×3, so it gives no proof or breaking loads for M9×1. For an estimate, multiply the tensile stress area of 51.0 mm² by the stress of the material: for example, 800 MPa gives 40.8 kN.
M9 pitches compared
All standard pitches on the M9 diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | Pitch (mm) | Pitch Ø (mm) | Minor Ø D1 (mm) | Stress area (mm²) | Tap drill |
|---|---|---|---|---|---|
| M9×1.25 | 1.25 | 8.188 | 7.647 | 48.1 | 7.8 mm |
| M9×1 | 1 | 8.350 | 7.917 | 51.0 | 8 mm |
Closest inch thread
No inch thread is interchangeable with M9×1. The nearest is 3/8-24 UNF, which is 0.52 mm larger in diameter with a pitch of 1.058 mm against 1.000 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
| Thread | Major Ø (mm) | Pitch (mm) | TPI | Ø difference (mm) |
|---|---|---|---|---|
| M9×1 (this thread) | 9.000 | 1.000 | 25.40 | — |
| 3/8-24 UNF | 9.525 | 1.058 | 24.00 | +0.525 |
| 3/8-32 UNEF | 9.525 | 0.794 | 32.00 | +0.525 |
| 3/8-16 UNC | 9.525 | 1.587 | 16.00 | +0.525 |
How the M9×1 dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 1 mm
- H = 0.866025 × P = 0.866 mm
- d2 = d − 0.649519 × P = 9 − 0.650 = 8.350 mm
- D1 = d − 1.082532 × P = 9 − 1.083 = 7.917 mm
- d3 = d − 1.226869 × P = 9 − 1.227 = 7.773 mm (external root, 17H/24 deep)
- As = π/4 × ((d2 + d3) ÷ 2)² = π/4 × 8.062² = 51.05 mm²
- Lead angle = arctan(P ÷ (π × d2)) = 2.18°
Related sizes and tools
Sources: ISO 68-1 and ISO 724 (profile and basic dimensions), ISO 261 (diameter and pitch series), ISO 965-1 (tolerances), ISO 273 (clearance holes), ISO 898-1 (mechanical properties) and tap drill tables based on ISO 2306. How this site checks its data. Last reviewed 29 September 2026.