5/16-24 Tap Drill Size and Drill Bit Size
Use a I drill bit
5/16-24 · 0.272 in / 6.91 mm · ~75% thread
5/16-24 is the UNF fine version of the 5/16 inch thread. A letter I bit gives correct fine-thread drill sizing.
At a glance
| Tap drill | I |
|---|---|
| Inches | 0.272 in |
| Millimetres | 6.91 mm |
| Major diameter | 0.3125 in |
| Pitch | 0.0417 in |
| Threads per inch | 24 |
Why this size
drillØ = major − (pitch × %thread) = 0.3125 − (0.0417 × 0.75) = 0.2812 in → next standard drill I (0.272 in / 6.91 mm)
Compare to 5/16-18 coarse (a letter F bit). The 5/16-24 fine thread gets the tighter letter I drill.
Clearance holes for 5/16-24
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | I |
|---|---|
| Close fit clearance | 21/64 bit |
| Free fit clearance | 23/64 bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: