M30×2 Tap Drill Size
The number you came for, then the reasoning behind it.
M30×2 tap drill — 75% thread
28 mm
28.051 mm exact · nearest inch drill 71/64 (1.1094")
How it's calculated
Tap drill = major diameter − pitch → 30 − 2 = 28 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% | 28.701 mm | 28.5 mm | 1.1250" (9/8 drill) |
| 65% | 28.311 mm | 28.5 mm | 1.1250" (9/8 drill) |
| 75% | 28.051 mm | 28 mm | 1.1094" (71/64 drill) |
| 85% | 27.792 mm | 28 mm | 1.1094" (71/64 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.