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1/2-13 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1/2-13 UNC tap drill — 75% thread
27/64
0.4219" · 10.8 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.5000 − (0.01299 × 75) / 13 = 0.4251"
The nearest standard drill above that is 27/64 at 0.4219".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.4500" | 29/64 (0.4531") | 11.4 mm |
| 65% | 0.4350" | 7/16 (0.4375") | 11.1 mm |
| 75% | 0.4251" | 27/64 (0.4219") | 10.8 mm |
| 85% | 0.4151" | Z (0.4130") | 10.5 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.