3/8-24 Tap Drill Size and Correct Drill Bit
Use a Q drill bit
3/8-24 · 0.332 in / 8.43 mm · ~75% thread
3/8-24 is the UNF fine version of the 3/8 inch thread. A letter Q bit gives the correct fine-thread tap drill.
At a glance
| Tap drill | Q |
|---|---|
| Inches | 0.332 in |
| Millimetres | 8.43 mm |
| Major diameter | 0.375 in |
| Pitch | 0.0417 in |
| Threads per inch | 24 |
Why this size
drillØ = major − (pitch × %thread) = 0.375 − (0.0417 × 0.75) = 0.3437 in → next standard drill Q (0.332 in / 8.43 mm)
For 3/8-16 coarse use a 5/16 in bit. The 3/8-24 fine thread uses the tighter letter Q drill.
Clearance holes for 3/8-24
A clearance hole lets the bolt pass through unthreaded – close fit keeps parts aligned, free fit allows movement.
| Tap drill (cuts threads) | Q |
|---|---|
| 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: