7/8-9 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
7/8-9 UNC tap drill — 75% thread
49/64
0.7656" · 19.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.8750 − (0.01299 × 75) / 9 = 0.7668"
The nearest standard drill above that is 49/64 at 0.7656".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.8028" | 51/64 (0.7969") | 20.4 mm |
| 65% | 0.7812" | 25/32 (0.7812") | 19.8 mm |
| 75% | 0.7668" | 49/64 (0.7656") | 19.5 mm |
| 85% | 0.7523" | 3/4 (0.7500") | 19.1 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.