#4-40 Tap Drill Size & Thread Specs
Everything for a #4-40 UNC thread on one page: tap drill (#43), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · #4-40
#4-40 specs
SizePlate
#4-40 UNC · tap with
#43 drill
.0890″
2.26 mm · 71% thread
- Metric drill
- 2.3 mm · .0906
- Clearance
- #32 / #30
- Wrench
- 1/4″
- Cap screw key
- 3/32″
Thread
- Nominal size
- #4 · 2.84 mm
- Major diameter
- .1120″
- Threads per inch
- 40
- Pitch
- .0250″ · 0.635 mm
- Series
- UNC (coarse)
- UNF version
- #4-48
Drilling
- Tap drill (75%)
- #43 · .0890
- Metric substitute
- 2.3 mm · .0906
- Tap drill (≈50%)
- #41 · .0960
- Close fit
- #32 · .1160
- Free fit
- #30 · .1285
Tools
- Hex bolt wrench
- —
- Hex nut wrench
- 1/4″
- Socket head cap key
- 3/32″
- Button head key
- 1/16″
- Flat head key
- 1/16″
Nearest metric
- Thread
- M3 × 0.5
- Major diameter
- 3 mm
- Pitch
- 0.5 mm
- Tap drill
- 2.5 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 |
|---|---|---|---|
| #44 | .0860 | 2.18 | 80% |
| 2.2 mm | .0866 | 2.2 | 78% |
| #43 | .0890 | 2.26 | 71% |
| 2.3 mm | .0906 | 2.3 | 66% |
| #42 | .0935 | 2.37 | 57% |
| 3/32 | .0938 | 2.38 | 56% |
| 2.4 mm | .0945 | 2.4 | 54% |
| #41 | .0960 | 2.44 | 49% |
Neighboring sizes
Questions
What does #4-40 mean?
A #4 nominal diameter with 40 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is #4-48.
Can I use a 2.3 mm drill instead of #43?
Usually. 2.3 mm is .0906″ against .0890″ for #43, 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.