1/4-20 Tap Drill Size and Drill Bit Guide
Use a #7 drill bit
1/4-20 · 0.201 in / 5.105 mm · ~75% thread
A 1/4-20 tap is the most common coarse machine screw thread in American hardware. Drill a #7 hole before tapping for around 75% thread engagement.
At a glance
| Tap drill | #7 |
|---|---|
| Inches | 0.201 in |
| Millimetres | 5.105 mm |
| Major diameter | 0.25 in |
| Pitch | 0.05 in |
| Threads per inch | 20 |
Why this size
drillØ = major − (pitch × %thread) = 0.25 − (0.05 × 0.75) = 0.2125 in → next standard drill #7 (0.201 in / 5.105 mm)
Use a #7 for the standard 75% fit. A 13/64 inch bit (0.2031 in) is slightly looser and safer for softer aluminium.
Clearance holes for 1/4-20
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | #7 |
|---|---|
| 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: