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1-1/8-7 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1-1/8-7 UNC tap drill — 75% thread
63/64
0.9844" · 25 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.1250 − (0.01299 × 75) / 7 = 0.9858"
The nearest standard drill above that is 63/64 at 0.9844".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 1.0322" | 33/32 (1.0312") | 26 mm |
| 65% | 1.0044" | 1 (1.0000") | 25.5 mm |
| 75% | 0.9858" | 63/64 (0.9844") | 25 mm |
| 85% | 0.9673" | 31/32 (0.9688") | 24.6 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.