#8-32 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
#8-32 UNC tap drill — 75% thread
#29
0.1360" · 3.4 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.1640 − (0.01299 × 75) / 32 = 0.1336"
The nearest standard drill above that is #29 at 0.1360".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.1437" | #27 (0.1440") | 3.7 mm |
| 65% | 0.1376" | #29 (0.1360") | 3.5 mm |
| 75% | 0.1336" | #29 (0.1360") | 3.4 mm |
| 85% | 0.1295" | #30 (0.1285") | 3.3 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.