M30×3.5 Tap Drill Size
The number you came for, then the reasoning behind it.
M30×3.5 tap drill — 75% thread
26.5 mm
26.590 mm exact · nearest inch drill 67/64 (1.0469")
How it's calculated
Tap drill = major diameter − pitch → 30 − 3.5 = 26.5 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% | 27.727 mm | 27.5 mm | 1.0781" (69/64 drill) |
| 65% | 27.045 mm | 27 mm | 1.0625" (17/16 drill) |
| 75% | 26.590 mm | 26.5 mm | 1.0469" (67/64 drill) |
| 85% | 26.135 mm | 26 mm | 1.0312" (33/32 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.