M5×0.8 Tap Drill Size
The number you came for, then the reasoning behind it.
M5×0.8 tap drill — 75% thread
4.2 mm
4.221 mm exact · nearest inch drill #19 (0.1660")
How it's calculated
Tap drill = major diameter − pitch → 5 − 0.8 = 4.2 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% | 4.480 mm | 4.5 mm | 0.1770" (#16 drill) |
| 65% | 4.324 mm | 4.3 mm | 0.1695" (#18 drill) |
| 75% | 4.221 mm | 4.2 mm | 0.1660" (#19 drill) |
| 85% | 4.117 mm | 4.1 mm | 0.1610" (#20 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.