#6-40 Tap Drill Size & Thread Specs
Everything for a #6-40 UNF thread on one page: tap drill (#33), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · #6-40
#6-40 specs
SizePlate
#6-40 UNF · tap with
#33 drill
.1130″
2.87 mm · 77% thread
- Metric drill
- 2.9 mm · .1142
- Clearance
- #27 / #25
- Wrench
- 5/16″
- Cap screw key
- 7/64″
Thread
- Nominal size
- #6 · 3.51 mm
- Major diameter
- .1380″
- Threads per inch
- 40
- Pitch
- .0250″ · 0.635 mm
- Series
- UNF (fine)
- UNC version
- #6-32
Drilling
- Tap drill (75%)
- #33 · .1130
- Metric substitute
- 2.9 mm · .1142
- Tap drill (≈50%)
- #31 · .1200
- Close fit
- #27 · .1440
- Free fit
- #25 · .1495
Tools
- Hex bolt wrench
- —
- Hex nut wrench
- 5/16″
- Socket head cap key
- 7/64″
- Button head key
- 5/64″
- Flat head key
- 5/64″
Nearest metric
- Thread
- M4 × 0.7
- Major diameter
- 4 mm
- Pitch
- 0.7 mm
- Tap drill
- 3.3 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 |
|---|---|---|---|
| 2.8 mm | .1102 | 2.8 | 85% |
| #34 | .1110 | 2.82 | 83% |
| #33 | .1130 | 2.87 | 77% |
| 2.9 mm | .1142 | 2.9 | 73% |
| #32 | .1160 | 2.95 | 68% |
| 3 mm | .1181 | 3 | 61% |
| #31 | .1200 | 3.05 | 55% |
| 3.1 mm | .1220 | 3.1 | 49% |
Neighboring sizes
Questions
What does #6-40 mean?
A #6 nominal diameter with 40 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is #6-32.
Can I use a 2.9 mm drill instead of #33?
Usually. 2.9 mm is .1142″ against .1130″ for #33, 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.