#3-56 tap drill size
The tap drill for a #3-56 UNF thread is a #45 drill (0.0820 in, 2.08 mm). It leaves about 73 % thread. The closest metric drill is 2.1 mm, which leaves 70 % thread.
Choose a drill
The standard drill plus the closest metric sizes. Each option redraws the section above.
#3-56 thread data
- Thread
- #3-56 UNF
- Threads per inch
- 56 pitch 0.454 mm
- Major diameter
- 0.0990" 2.51 mm
- Tap drill
- #45 0.0820"
- In millimetres
- 2.08 mm
- Thread engagement
- 73 %
- UNC thread
- #3-48 #47 drill
How much thread you need
Thread percentage to aim for by material.
Bars run from 50 % to 90 % thread. Typical shop practice for cut taps. Harder, tougher materials take less thread to save the tap; soft materials need more engagement to hold.
How the percentage is worked out
- TPI
- 56 threads per inch
- D
- 0.0990", major diameter
- drill
- 0.0820" (#45)
Why not 100 %?
Going from 75 % to 100 % thread adds only about 5 % strength but roughly triples the torque needed to turn the tap, so the standard drill leaves 65–80 %.
No number or letter drill in the shop? The metric drills above are the closest substitutes, with the thread percentage each one leaves.
Other UNF threads
Common questions
What size drill for a #3-56 tap?
Use a #45 drill (0.0820 in, 2.08 mm). It leaves about 73 % thread.
Can I use a metric drill for #3-56?
Yes. A 2.1 mm drill gives about 70 % thread.