ISO metric · coarse
M22 × 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
20 mm
0.7874" · 62% thread
Or 25/32 (0.7813") for 66%
Form tap
21 mm
0.8268" · 59% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 22.000 mm (0.8661")
- Pitch
- 2.5 mm
- Pitch Ø (basic)
- 20.376 mm (0.8022")
- Minor Ø (internal, basic)
- 19.294 mm (0.7596")
- 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 |
|---|---|---|---|
| 49/64 | 0.7656 | 19.45 | 79% |
| 19.5 mm | 0.7677 | 19.50 | 77% |
| 25/32suggested | 0.7813 | 19.84 | 66% |
| 20 mm | 0.7874 | 20.00 | 62% |
| 51/64 | 0.7969 | 20.24 | 54% |
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 |
|---|---|---|---|
| 13/16 | 0.8125 | 20.64 | 80% |
| 21 mm | 0.8268 | 21.00 | 59% |
| 53/64 | 0.8281 | 21.03 | 57% |
3-wire measurement
- Best wire size
- 1.443 mm (0.0568")
- Over wires, at basic pitch Ø
- 22.541 mm (0.8875")
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