3/8-16 Tap Drill Size and Correct Drill Bit
Use a 5/16 drill bit
3/8-16 · 0.3125 in / 7.94 mm · ~75% thread
3/8-16 is a common coarse UNC thread in hardware, automotive and project work. A 5/16 inch bit is the correct tap drill.
At a glance
| Tap drill | 5/16 |
|---|---|
| Inches | 0.3125 in |
| Millimetres | 7.94 mm |
| Major diameter | 0.375 in |
| Pitch | 0.0625 in |
| Threads per inch | 16 |
Why this size
drillØ = major − (pitch × %thread) = 0.375 − (0.0625 × 0.75) = 0.3281 in → next standard drill 5/16 (0.3125 in / 7.94 mm)
For a lighter friction fit in aluminium, a 21/64 bit (0.3281 in) is a good looser option. 5/16 in is the standard tap drill.
Clearance holes for 3/8-16
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | 5/16 |
|---|---|
| Close fit clearance | 25/64 bit |
| Free fit clearance | 27/64 bit |
Related sizes
Compare with adjacent threads – coarse, fine and the opposite measurement system: