ISO metric · coarse
M18 × 2.5 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
16 mm
0.6299" · 62% thread
Or 5/8 (0.6250") for 65%
Form tap
17 mm
0.6693" · 59% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 18.000 mm (0.7087")
- Pitch
- 2.5 mm
- Pitch Ø (basic)
- 16.376 mm (0.6447")
- Minor Ø (internal, basic)
- 15.294 mm (0.6021")
- Thread depth (internal)
- 1.353 mm (0.0533")
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 |
|---|---|---|---|
| 39/64 | 0.6094 | 15.48 | 78% |
| 15.5 mm | 0.6102 | 15.50 | 77% |
| 5/8suggested | 0.6250 | 15.88 | 65% |
| 16 mm | 0.6299 | 16.00 | 62% |
| 41/64 | 0.6406 | 16.27 | 53% |
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 |
|---|---|---|---|
| 21/32 | 0.6563 | 16.67 | 78% |
| 17 mm | 0.6693 | 17.00 | 59% |
| 43/64 | 0.6719 | 17.07 | 55% |
3-wire measurement
- Best wire size
- 1.443 mm (0.0568")
- Over wires, at basic pitch Ø
- 18.541 mm (0.7300")
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