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#10-24 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
#10-24 UNC tap drill — 75% thread
#25
0.1495" · 3.8 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.1900 − (0.01299 × 75) / 24 = 0.1494"
The nearest standard drill above that is #25 at 0.1495".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1629" | #20 (0.1610") | 4.1 mm |
| 65% | 0.1548" | #23 (0.1540") | 3.9 mm |
| 75% | 0.1494" | #25 (0.1495") | 3.8 mm |
| 85% | 0.1440" | #27 (0.1440") | 3.7 mm |
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.