Home › 7/8-14 UNF › 7/8-14 UNF tap drill size
7/8-14 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
7/8-14 UNF tap drill — 75% thread
13/16
0.8125" · 20.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.8750 − (0.01299 × 75) / 14 = 0.8054"
The nearest standard drill above that is 13/16 at 0.8125".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.8286" | 53/64 (0.8281") | 21 mm |
| 65% | 0.8147" | 13/16 (0.8125") | 20.7 mm |
| 75% | 0.8054" | 13/16 (0.8125") | 20.5 mm |
| 85% | 0.7961" | 51/64 (0.7969") | 20.2 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.