10-32 Tap Drill Size and Correct Drill Bit
Use a #21 drill bit
10-32 · 0.159 in / 4.04 mm · ~75% thread
10-32 is a small UNF (fine) machine screw thread used in electronics and precision work. A #21 drill gives the correct 75% thread before tapping.
At a glance
| Tap drill | #21 |
|---|---|
| Inches | 0.159 in |
| Millimetres | 4.04 mm |
| Major diameter | 0.19 in |
| Pitch | 0.0313 in |
| Threads per inch | 32 |
Why this size
drillØ = major − (pitch × %thread) = 0.19 − (0.0313 × 0.75) = 0.1665 in → next standard drill #21 (0.159 in / 4.04 mm)
10-32 is UNF fine. For coarse 10-24 use a #25 bit. The tighter pitch on 10-32 means a finer, stronger thread.
Clearance holes for 10-32
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | #21 |
|---|---|
| Close fit clearance | #5 bit |
| Free fit clearance | B bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: