3/8-24 tap drill size
Letter Q
- Inches
- 0.3320
- Millimetres
- 8.43
- Thread
- 79%
Drill a Letter Q hole before cutting a 3/8-24 thread. That is the standard tap drill for about 75% thread.
3/8-24 thread and hole sizes
| Thread | 3/8 diameter, 24 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.3750 in 9.52 mm |
| Pitch | 0.0417 in 1.058 mm |
| Tap drill, about 75% thread | Letter Q 0.3320 in |
| Tap drill, about 50% thread | Letter S 0.3480 in, for stainless and other hard materials |
| Clearance hole, close fit | Letter W 0.3860 in |
| Clearance hole, free fit | Letter X 0.3970 in |
| Nearest metric bit | 8.4 mm |
Why Letter Q
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 3/8-24 thread the hole that leaves exactly 75% thread is 0.3344 in. The closest standard bit is Letter Q at 0.3320 in, which leaves 79%.
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
- 3/8-24 tap
- Letter Q drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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