#3-56 Tap Drill Size & Thread Specs
Everything for a #3-56 UNF thread on one page: tap drill (#45), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · #3-56
#3-56 specs
SizePlate
#3-56 UNF · tap with
#45 drill
.0820″
2.08 mm · 73% thread
- Metric drill
- 2.1 mm · .0827
- Clearance
- #37 / #35
Thread
- Nominal size
- #3 · 2.51 mm
- Major diameter
- .0990″
- Threads per inch
- 56
- Pitch
- .0179″ · 0.454 mm
- Series
- UNF (fine)
- UNC version
- #3-48
Drilling
- Tap drill (75%)
- #45 · .0820
- Metric substitute
- 2.1 mm · .0827
- Tap drill (≈50%)
- #44 · .0860
- Close fit
- #37 · .1040
- Free fit
- #35 · .1100
Nearest metric
- Thread
- M2.5 × 0.45
- Major diameter
- 2.5 mm
- Pitch
- 0.45 mm
- Tap drill
- 2.05 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 |
|---|---|---|---|
| 2.05 mm | .0807 | 2.05 | 79% |
| #46 | .0810 | 2.06 | 78% |
| #45 | .0820 | 2.08 | 73% |
| 2.1 mm | .0827 | 2.1 | 70% |
| #44 | .0860 | 2.18 | 56% |
| 2.2 mm | .0866 | 2.2 | 53% |
Neighboring sizes
Questions
What does #3-56 mean?
A #3 nominal diameter with 56 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is #3-48.
Can I use a 2.1 mm drill instead of #45?
Usually. 2.1 mm is .0827″ against .0820″ for #45, 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.