M4×0.7 Tap Drill Size
The number you came for, then the reasoning behind it.
M4×0.7 tap drill — 75% thread
3.3 mm
3.318 mm exact · nearest inch drill #30 (0.1285")
How it's calculated
Tap drill = major diameter − pitch → 4 − 0.7 = 3.3 mm
The exact 75% figure is major − (pitch × 75 / 76.98). The simple subtract-the-pitch rule lands within a hundredth of a millimetre of it, which is why every machinist uses it.
All engagement percentages
| % thread | Exact hole | Metric drill | Inch equivalent |
|---|---|---|---|
| 50% | 3.545 mm | 3.5 mm | 0.1360" (#29 drill) |
| 65% | 3.409 mm | 3.4 mm | 0.1360" (#29 drill) |
| 75% | 3.318 mm | 3.3 mm | 0.1285" (#30 drill) |
| 85% | 3.227 mm | 3.2 mm | 0.1250" (1/8 drill) |
Which percentage should you actually use?
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.