#6-40 tap drill size
#33
- Inches
- 0.1130
- Millimetres
- 2.87
- Thread
- 77%
Drill a #33 hole before cutting a #6-40 thread. That is the standard tap drill for about 75% thread.
#6-40 thread and hole sizes
| Thread | #6 diameter, 40 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.1380 in 3.51 mm |
| Pitch | 0.0250 in 0.635 mm |
| Tap drill, about 75% thread | #33 0.1130 in |
| Tap drill, about 50% thread | #31 0.1200 in, for stainless and other hard materials |
| Clearance hole, close fit | #27 0.1440 in |
| Clearance hole, free fit | #25 0.1495 in |
| Nearest metric bit | 2.9 mm |
Why #33
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #6-40 thread the hole that leaves exactly 75% thread is 0.1136 in. The closest standard bit is #33 at 0.1130 in, which leaves 77%.
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
- #6-40 tap
- #33 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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