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1-1/4-7 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1-1/4-7 UNC tap drill — 75% thread
71/64
1.1094" · 28 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.2500 − (0.01299 × 75) / 7 = 1.1108"
The nearest standard drill above that is 71/64 at 1.1094".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 1.1572" | 37/32 (1.1562") | 29.5 mm |
| 65% | 1.1294" | 9/8 (1.1250") | 28.5 mm |
| 75% | 1.1108" | 71/64 (1.1094") | 28 mm |
| 85% | 1.0923" | 35/32 (1.0938") | 27.5 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.