5/16-24 tap drill size
Letter I
- Inches
- 0.2720
- Millimetres
- 6.91
- Thread
- 75%
Drill a Letter I hole before cutting a 5/16-24 thread. That is the standard tap drill for about 75% thread.
5/16-24 thread and hole sizes
| Thread | 5/16 diameter, 24 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.3125 in 7.94 mm |
| Pitch | 0.0417 in 1.058 mm |
| Tap drill, about 75% thread | Letter I 0.2720 in |
| Tap drill, about 50% thread | 9/32" 0.2812 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 | 6.9 mm |
Why Letter I
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 5/16-24 thread the hole that leaves exactly 75% thread is 0.2719 in. The closest standard bit is Letter I at 0.2720 in, which leaves 75%.
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
- 5/16-24 tap
- Letter I drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
As an Amazon Associate we earn from qualifying purchases. Details