5/16-18 tap drill size
Letter F
- Inches
- 0.2570
- Millimetres
- 6.53
- Thread
- 77%
Drill a Letter F hole before cutting a 5/16-18 thread. That is the standard tap drill for about 75% thread.
No Letter F bit? A 17/64" bit (0.2656 in) gives about 65% thread, which holds fine in most materials.
5/16-18 thread and hole sizes
| Thread | 5/16 diameter, 18 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.3125 in 7.94 mm |
| Pitch | 0.0556 in 1.411 mm |
| Tap drill, about 75% thread | Letter F 0.2570 in |
| Tap drill, about 50% thread | Letter J 0.2770 in, for stainless and other hard materials |
| Clearance hole, close fit | Letter P 0.3230 in |
| Clearance hole, free fit | Letter Q 0.3320 in |
| Nearest metric bit | 6.5 mm |
Why Letter F
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 5/16-18 thread the hole that leaves exactly 75% thread is 0.2584 in. The closest standard bit is Letter F at 0.2570 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
- 5/16-18 tap
- Letter F drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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