M14×1.25 Thread Dimensions and Tap Drill
M14×1.25 is an ISO metric fine-pitch thread: the 14 mm diameter with a 1.25 mm pitch (20.3 threads per inch) instead of the coarse 2 mm of M14×2. The thread is 0.41 mm shallower, so the tensile stress area rises from 115 to 129 mm² (12% more) and the lead angle falls from 2.87° to 1.73°.
ISO 261 lists M14×1.25 for spark plugs only.
M14×1.25 is the long-standing spark plug thread, which is the only use ISO 261 lists it for.
- Tap drill12.8 mm74% thread
- Pitch1.25 mm20.3 TPI
- Major diameter14.000 mm0.5512 in
- Pitch diameter13.188 mm0.5192 in
- Minor diameter (nut)12.647 mm0.4979 in
- Tensile stress area129 mm²0.200 in²
- Clearance hole15.5 mmmedium fit, ISO 273
Thread profile
M14×1.25 basic dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 14.000 | 0.5512 |
| Pitch P | 1.250 | 0.0492 |
| Pitch diameter d2 = D2 | 13.188 | 0.5192 |
| Minor diameter, internal thread D1 | 12.647 | 0.4979 |
| Minor diameter, external thread d3 | 12.466 | 0.4908 |
| Height of fundamental triangle H | 1.083 | 0.0426 |
| Thread height, internal (5H/8) | 0.677 | 0.0266 |
| Thread height, external (17H/24) | 0.767 | 0.0302 |
| Crest flat of external thread (P/8) | 0.156 | 0.0062 |
| Crest flat of internal thread (P/4) | 0.312 | 0.0123 |
| Property | Value |
|---|---|
| Threads per inch | 20.32 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 1.73° |
| Tensile stress area As | 129 mm² (0.200 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.
M14×1.25 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 28 µm, so even at its largest it sits 0.028 mm below the basic size and leaves room for a thin plating.
| External thread 6g | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Major diameter d | 13.972 | 13.760 | 0.212 |
| Pitch diameter d2 | 13.160 | 13.028 | 0.132 |
| Internal thread 6H | Max (mm) | Min (mm) | Tolerance (mm) |
|---|---|---|---|
| Minor diameter D1 | 12.912 | 12.647 | 0.265 |
| Pitch diameter D2 | 13.368 | 13.188 | 0.180 |
Computed from the ISO 965-1 deviations and tolerances for normal length of engagement; the results match the limits tabulated in ISO 965-2.
M14×1.25 tap drill size
Tap drill for M14×1.25: 12.8 mm
This leaves about 74% thread in the tapped hole.
The rule for metric tap drills is diameter minus pitch: 14 − 1.25 = 12.75 mm. Tap drill tables round this to 12.8 mm, a stocked drill size.
The 6H minor diameter of the finished nut thread must lie between 12.647 and 12.912 mm. The recommended drill falls inside that band.
Other drills that work for M14×1.25
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 |
|---|---|---|---|---|---|
| 12.6 mm | 12.60 | 0.4961 | 86% | no | |
| 12.7 mm | 12.70 | 0.5000 | 80% | yes | |
| 1/2" (0.5000 in) | 12.70 | 0.5000 | 80% | yes | |
| 12.75 mm | 12.75 | 0.5020 | 77% | yes | |
| 12.8 mmrecommended | 12.80 | 0.5039 | 74% | yes | |
| 12.9 mm | 12.90 | 0.5079 | 68% | yes | |
| 13 mm | 13.00 | 0.5118 | 62% | no | |
| 13.1 mm | 13.10 | 0.5157 | 55% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for M14
The clearance hole depends only on the 14 mm diameter, not on the pitch. The medium series, 15.5 mm, is the usual choice for general assembly.
| Series | Hole Ø (mm) | Hole Ø (in) | Clearance on Ø (mm) |
|---|---|---|---|
| Fine (H12) | 15 | 0.5906 | 1 |
| Medium (H13) | 15.5 | 0.6102 | 1.5 |
| Coarse (H14) | 16.5 | 0.6496 | 2.5 |
Hole diameters from ISO 273.
M14×1.25 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 M14×1.25. For an estimate, multiply the tensile stress area of 129 mm² by the stress of the material: for example, 800 MPa gives 103 kN.
M14 pitches compared
All standard pitches on the M14 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.
Closest inch thread
No inch thread is interchangeable with M14×1.25. The nearest is 9/16-18 UNF, which is 0.29 mm larger in diameter with a pitch of 1.411 mm against 1.250 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) |
|---|---|---|---|---|
| M14×1.25 (this thread) | 14.000 | 1.250 | 20.32 | — |
| 9/16-18 UNF | 14.287 | 1.411 | 18.00 | +0.287 |
| 9/16-24 UNEF | 14.287 | 1.058 | 24.00 | +0.287 |
| 9/16-12 UNC | 14.287 | 2.117 | 12.00 | +0.287 |
How the M14×1.25 dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 1.25 mm
- H = 0.866025 × P = 1.083 mm
- d2 = d − 0.649519 × P = 14 − 0.812 = 13.188 mm
- D1 = d − 1.082532 × P = 14 − 1.353 = 12.647 mm
- d3 = d − 1.226869 × P = 14 − 1.534 = 12.466 mm (external root, 17H/24 deep)
- As = π/4 × ((d2 + d3) ÷ 2)² = π/4 × 12.827² = 129.23 mm²
- Lead angle = arctan(P ÷ (π × d2)) = 1.73°
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.