M20×1.5 Tap Drill Size
The number you came for, then the reasoning behind it.
M20×1.5 tap drill — 75% thread
18.5 mm
18.539 mm exact · nearest inch drill 47/64 (0.7344")
How it's calculated
Tap drill = major diameter − pitch → 20 − 1.5 = 18.5 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% | 19.026 mm | 19 mm | 0.7500" (3/4 drill) |
| 65% | 18.733 mm | 18.7 mm | 0.7344" (47/64 drill) |
| 75% | 18.539 mm | 18.5 mm | 0.7344" (47/64 drill) |
| 85% | 18.344 mm | 18.3 mm | 0.7188" (23/32 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.