#10-32 tap drill size
#21
- Inches
- 0.1590
- Millimetres
- 4.04
- Thread
- 76%
Drill a #21 hole before cutting a #10-32 thread. That is the standard tap drill for about 75% thread.
#10-32 thread and hole sizes
| Thread | #10 diameter, 32 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.1900 in 4.83 mm |
| Pitch | 0.0312 in 0.794 mm |
| Tap drill, about 75% thread | #21 0.1590 in |
| Tap drill, about 50% thread | #18 0.1695 in, for stainless and other hard materials |
| Clearance hole, close fit | #9 0.1960 in |
| Clearance hole, free fit | #7 0.2010 in |
| Nearest metric bit | 4 mm |
Why #21
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #10-32 thread the hole that leaves exactly 75% thread is 0.1596 in. The closest standard bit is #21 at 0.1590 in, which leaves 76%.
More thread than that adds almost no strength and makes the tap much easier to break. In hard or tough material, go one step larger.
Tools for this job
- #10-32 tap
- #21 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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