M1.6×0.35 Tap Drill Size
The number you came for, then the reasoning behind it.
M1.6×0.35 tap drill — 75% thread
1.25 mm
1.259 mm exact · nearest inch drill 3/64 (0.0469")
How it's calculated
Tap drill = major diameter − pitch → 1.6 − 0.35 = 1.25 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% | 1.373 mm | 1.4 mm | 0.0550" (#54 drill) |
| 65% | 1.304 mm | 1.3 mm | 0.0520" (#55 drill) |
| 75% | 1.259 mm | 1.25 mm | 0.0469" (3/64 drill) |
| 85% | 1.214 mm | 1.2 mm | 0.0469" (3/64 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.