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7/16-20 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
7/16-20 UNF tap drill — 75% thread
25/64
0.3906" · 9.9 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.4375 − (0.01299 × 75) / 20 = 0.3888"
The nearest standard drill above that is 25/64 at 0.3906".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.4050" | Y (0.4040") | 10.3 mm |
| 65% | 0.3953" | X (0.3970") | 10 mm |
| 75% | 0.3888" | 25/64 (0.3906") | 9.9 mm |
| 85% | 0.3823" | W (0.3860") | 9.7 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.