1-8 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1-8 UNC tap drill — 75% thread
7/8
0.8750" · 22.3 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.0000 − (0.01299 × 75) / 8 = 0.8782"
The nearest standard drill above that is 7/8 at 0.8750".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.9188" | 59/64 (0.9219") | 23.3 mm |
| 65% | 0.8945" | 57/64 (0.8906") | 22.7 mm |
| 75% | 0.8782" | 7/8 (0.8750") | 22.3 mm |
| 85% | 0.8620" | 55/64 (0.8594") | 21.9 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.