#5-40 tap drill size
#38
- Inches
- 0.1015
- Millimetres
- 2.58
- Thread
- 72%
Drill a #38 hole before cutting a #5-40 thread. That is the standard tap drill for about 75% thread.
#5-40 thread and hole sizes
| Thread | #5 diameter, 40 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.1250 in 3.17 mm |
| Pitch | 0.0250 in 0.635 mm |
| Tap drill, about 75% thread | #38 0.1015 in |
| Tap drill, about 50% thread | 7/64" 0.1094 in, for stainless and other hard materials |
| Clearance hole, close fit | #30 0.1285 in |
| Clearance hole, free fit | #29 0.1360 in |
| Nearest metric bit | 2.6 mm |
Why #38
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #5-40 thread the hole that leaves exactly 75% thread is 0.1006 in. The closest standard bit is #38 at 0.1015 in, which leaves 72%.
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
- #5-40 tap
- #38 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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