#4-40 tap drill size
#43
- Inches
- 0.0890
- Millimetres
- 2.26
- Thread
- 71%
Drill a #43 hole before cutting a #4-40 thread. That is the standard tap drill for about 75% thread.
#4-40 thread and hole sizes
| Thread | #4 diameter, 40 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.1120 in 2.84 mm |
| Pitch | 0.0250 in 0.635 mm |
| Tap drill, about 75% thread | #43 0.0890 in |
| Tap drill, about 50% thread | #41 0.0960 in, for stainless and other hard materials |
| Clearance hole, close fit | #32 0.1160 in |
| Clearance hole, free fit | #30 0.1285 in |
| Nearest metric bit | 2.3 mm |
Why #43
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #4-40 thread the hole that leaves exactly 75% thread is 0.0876 in. The closest standard bit is #43 at 0.0890 in, which leaves 71%.
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
- #4-40 tap
- #43 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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