1/4-28 Tap Drill Size and Drill Bit Size
Use a #3 drill bit
1/4-28 · 0.213 in / 5.41 mm · ~75% thread
1/4-28 is the UNF fine version of the 1/4 inch thread, common in aerospace and tubing. A #3 bit gives the correct fine-thread drill size.
At a glance
| Tap drill | #3 |
|---|---|
| Inches | 0.213 in |
| Millimetres | 5.41 mm |
| Major diameter | 0.25 in |
| Pitch | 0.0357 in |
| Threads per inch | 28 |
Why this size
drillØ = major − (pitch × %thread) = 0.25 − (0.0357 × 0.75) = 0.2232 in → next standard drill #3 (0.213 in / 5.41 mm)
Compare with 1/4-20 coarse (a #7 bit). The 1/4-28 fine thread uses the smaller-pitch #3 drill.
Clearance holes for 1/4-28
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | #3 |
|---|---|
| Close fit clearance | 17/64 bit |
| Free fit clearance | 19/64 bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: