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#10-32 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
#10-32 UNF tap drill — 75% thread
#21
0.1590" · 4.1 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.1900 − (0.01299 × 75) / 32 = 0.1596"
The nearest standard drill above that is #21 at 0.1590".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1697" | #18 (0.1695") | 4.3 mm |
| 65% | 0.1636" | #19 (0.1660") | 4.2 mm |
| 75% | 0.1596" | #21 (0.1590") | 4.1 mm |
| 85% | 0.1555" | 5/32 (0.1562") | 3.9 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.