3/8-24 Tap Drill Size & Thread Specs
Everything for a 3/8-24 UNF thread on one page: tap drill (Q), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 3/8-24
3/8-24 specs
SizePlate
3/8-24 UNF · tap with
Q drill
.3320″
8.43 mm · 79% thread
- Metric drill
- 8.4 mm · .3307
- Clearance
- W / X
- Wrench
- 9/16″
- Cap screw key
- 5/16″
Thread
- Nominal size
- 3/8″ · 9.52 mm
- Major diameter
- .3750″
- Threads per inch
- 24
- Pitch
- .0417″ · 1.058 mm
- Series
- UNF (fine)
- UNC version
- 3/8-16
Drilling
- Tap drill (75%)
- Q · .3320
- Metric substitute
- 8.4 mm · .3307
- Tap drill (≈50%)
- S · .3480
- Close fit
- W · .3860
- Free fit
- X · .3970
Tools
- Hex bolt wrench
- 9/16″
- Hex nut wrench
- 9/16″
- Socket head cap key
- 5/16″
- Button head key
- 7/32″
- Flat head key
- 7/32″
Nearest metric
- Thread
- M10 × 1.5
- Major diameter
- 10 mm
- Pitch
- 1.5 mm
- Tap drill
- 8.5 mm
- Interchangeable
- No
Not interchangeable means the threads look close but won't run together: forcing one into the other strips both.
Drill size by thread percentage
Larger drill, less thread, easier tapping. 75% is the usual target.
| Drill | Decimal | mm | Thread |
|---|---|---|---|
| 8.4 mm | .3307 | 8.4 | 82% |
| Q | .3320 | 8.43 | 79% |
| 8.5 mm | .3346 | 8.5 | 75% |
| 8.6 mm | .3386 | 8.6 | 67% |
| R | .3390 | 8.61 | 67% |
| 8.7 mm | .3425 | 8.7 | 60% |
| 11/32 | .3438 | 8.73 | 58% |
| 8.8 mm | .3465 | 8.8 | 53% |
| S | .3480 | 8.84 | 50% |
| 8.9 mm | .3504 | 8.9 | 45% |
Neighboring sizes
Questions
What does 3/8-24 mean?
A 3/8″ nominal diameter with 24 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is 3/8-16.
Can I use a 8.4 mm drill instead of Q?
Usually. 8.4 mm is .3307″ against .3320″ for Q, which changes the thread engagement by a few percent at most. The thread-percentage table above shows exactly how much thread each nearby drill leaves.