M18×1.5 Tap Drill Size
The number you came for, then the reasoning behind it.
M18×1.5 tap drill — 75% thread
16.5 mm
16.539 mm exact · nearest inch drill 21/32 (0.6562")
How it's calculated
Tap drill = major diameter − pitch → 18 − 1.5 = 16.5 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% | 17.026 mm | 17 mm | 0.6719" (43/64 drill) |
| 65% | 16.733 mm | 16.7 mm | 0.6562" (21/32 drill) |
| 75% | 16.539 mm | 16.5 mm | 0.6562" (21/32 drill) |
| 85% | 16.344 mm | 16.3 mm | 0.6406" (41/64 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.