M2.5×0.45 Tap Drill Size
The number you came for, then the reasoning behind it.
M2.5×0.45 tap drill — 75% thread
2.05 mm
2.062 mm exact · nearest inch drill #46 (0.0810")
How it's calculated
Tap drill = major diameter − pitch → 2.5 − 0.45 = 2.05 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% | 2.208 mm | 2.2 mm | 0.0860" (#44 drill) |
| 65% | 2.120 mm | 2.1 mm | 0.0820" (#45 drill) |
| 75% | 2.062 mm | 2.05 mm | 0.0810" (#46 drill) |
| 85% | 2.003 mm | 2 mm | 0.0785" (#47 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.