ISO metric · fine
M14 × 1.25 tap drill size
For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.
Cut tap
12.9 mm
0.5079" · 68% thread
Form tap
13.5 mm
0.5315" · 59% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 14.000 mm (0.5512")
- Pitch
- 1.25 mm
- Pitch Ø (basic)
- 13.188 mm (0.5192")
- Minor Ø (internal, basic)
- 12.647 mm (0.4979")
- Thread depth (internal)
- 0.677 mm (0.0266")
Basic sizes of the 60° form. Tolerance classes (6H) set the limits around them.
Cut tap drills
For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 1/2 | 0.5000 | 12.70 | 80% |
| 12.7 mm | 0.5000 | 12.70 | 80% |
| 12.8 mm | 0.5039 | 12.80 | 74% |
| 12.9 mmsuggested | 0.5079 | 12.90 | 68% |
| 13 mm | 0.5118 | 13.00 | 62% |
| 33/64 | 0.5156 | 13.10 | 56% |
Form tap drills
A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 17/32 | 0.5313 | 13.49 | 60% |
| 13.5 mm | 0.5315 | 13.50 | 59% |
3-wire measurement
- Best wire size
- 0.722 mm (0.0284")
- Over wires, at basic pitch Ø
- 14.271 mm (0.5618")
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ISO 261 / ISO 262 — ISO general purpose metric screw threads: general plan and selected sizes
- OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
- Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
- ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
- NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size