ISO metric · fine
M27 × 2 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
25 mm
0.9843" · 77% thread
Form tap
26 mm
1.0236" · 74% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 27.000 mm (1.0630")
- Pitch
- 2 mm
- Pitch Ø (basic)
- 25.701 mm (1.0118")
- Minor Ø (internal, basic)
- 24.835 mm (0.9778")
- Thread depth (internal)
- 1.083 mm (0.0426")
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 |
|---|---|---|---|
| 25 mm | 0.9843 | 25.00 | 77% |
| 63/64 | 0.9844 | 25.00 | 77% |
| 1 | 1.0000 | 25.40 | 62% |
| 25.5 mm | 1.0039 | 25.50 | 58% |
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 |
|---|---|---|---|
| 26 mm | 1.0236 | 26.00 | 74% |
| 1-1/32 | 1.0313 | 26.19 | 59% |
3-wire measurement
- Best wire size
- 1.155 mm (0.0455")
- Over wires, at basic pitch Ø
- 27.433 mm (1.0800")
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