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1/4-28 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
1/4-28 UNF tap drill — 75% thread
#3
0.2130" · 5.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.2500 − (0.01299 × 75) / 28 = 0.2152"
The nearest standard drill above that is #3 at 0.2130".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.2268" | #1 (0.2280") | 5.8 mm |
| 65% | 0.2198" | 7/32 (0.2188") | 5.6 mm |
| 75% | 0.2152" | #3 (0.2130") | 5.5 mm |
| 85% | 0.2106" | #4 (0.2090") | 5.3 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.