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#12-24 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
#12-24 UNC tap drill — 75% thread
#16
0.1770" · 4.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.2160 − (0.01299 × 75) / 24 = 0.1754"
The nearest standard drill above that is #16 at 0.1770".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1889" | #12 (0.1890") | 4.8 mm |
| 65% | 0.1808" | #15 (0.1800") | 4.6 mm |
| 75% | 0.1754" | #16 (0.1770") | 4.5 mm |
| 85% | 0.1700" | #18 (0.1695") | 4.3 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.