M18×2.5 Tap Drill Size
The number you came for, then the reasoning behind it.
M18×2.5 tap drill — 75% thread
15.5 mm
15.564 mm exact · nearest inch drill 39/64 (0.6094")
How it's calculated
Tap drill = major diameter − pitch → 18 − 2.5 = 15.5 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% | 16.376 mm | 16.4 mm | 0.6406" (41/64 drill) |
| 65% | 15.889 mm | 15.9 mm | 0.6250" (5/8 drill) |
| 75% | 15.564 mm | 15.5 mm | 0.6094" (39/64 drill) |
| 85% | 15.240 mm | 15.2 mm | 0.5938" (19/32 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.