ISO metric · coarse
M33 × 3.5 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
30 mm
1.1811" · 66% thread
Form tap
31.5 mm
1.2402" · 63% thread
Or 1-15/64 (1.2344") for 69%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 33.000 mm (1.2992")
- Pitch
- 3.5 mm
- Pitch Ø (basic)
- 30.727 mm (1.2097")
- Minor Ø (internal, basic)
- 29.211 mm (1.1500")
- Thread depth (internal)
- 1.894 mm (0.0746")
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 |
|---|---|---|---|
| 1-5/32 | 1.1563 | 29.37 | 80% |
| 29.5 mm | 1.1614 | 29.50 | 77% |
| 1-11/64 | 1.1719 | 29.77 | 71% |
| 30 mmsuggested | 1.1811 | 30.00 | 66% |
| 1-3/16 | 1.1875 | 30.16 | 62% |
| 30.5 mm | 1.2008 | 30.50 | 55% |
| 1-13/64 | 1.2031 | 30.56 | 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 |
|---|---|---|---|
| 31 mm | 1.2205 | 31.00 | 84% |
| 1-15/64suggested | 1.2344 | 31.35 | 69% |
| 31.5 mm | 1.2402 | 31.50 | 63% |
| 1-1/4 | 1.2500 | 31.75 | 53% |
3-wire measurement
- Best wire size
- 2.021 mm (0.0796")
- Over wires, at basic pitch Ø
- 33.758 mm (1.3290")
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