The tap drill for a #2-56 UNC thread is a #50 drill (1.78 mm), which leaves about 69% of full thread with a cutting tap. The ideal hole for 75% thread is 0.0686 in, and #50 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.0744 in | #49 | 0.0730 in / 1.85 mm | 56% |
| 60% | 0.0721 in | #49 | 0.0730 in / 1.85 mm | 56% |
| 65% | 0.0709 in | #50 | 0.0700 in / 1.78 mm | 69% |
| 70% | 0.0698 in | #50 | 0.0700 in / 1.78 mm | 69% |
| 75% | 0.0686 in | #50 | 0.0700 in / 1.78 mm | 69% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 13 points of thread apart. #51 (0.0670 in) gives 82% and a stronger but harder-to-tap thread; #50 (0.0700 in) gives 69% and less tapping torque. On #2-56 UNC the choice is mostly about tap breakage.
#2-56 holds small electronics, instrument panels, model parts and eyeglass-scale hardware. Its basic pitch diameter is 0.0744 in and its tensile stress area is 0.0037 in². A bolt of this size passes through a 0.1020 in normal-fit clearance hole (ASME B18.2.8).
Example: #2-56 UNC at 75% thread
Result: #50 (0.0700 in), 69% thread
| Major (nominal) diameter | 0.0860 in / 2.18 mm |
|---|---|
| Pitch | 56 TPI (0.0179 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.0744 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.0667 in |
| Tensile stress area | 0.0037 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.0940 in / 2.39 mm |
| Normal | 0.1020 in / 2.59 mm |
| Loose | 0.1160 in / 2.95 mm |
All UNC sizes:
Use #50 (0.0700 in). It leaves 69% of full thread with a cutting tap; the ideal 75% hole is 0.0686 in. For an easier-to-tap 50% thread, #49 (0.0730 in) gives 56%.
#50 is 1.78 mm. The nearest metric drill to the 75% target is 1.75 mm (0.0689 in), which gives 74% thread.
A forming tap needs a bigger hole: 0.0781 in for 65% thread, so drill 5/64" (0.0781 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #2-56 UNC, ASME B18.2.8 gives close fit 0.0940 in, normal fit 0.1020 in and loose fit 0.1160 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.0037 in², from As = 0.7854 × (D − 0.9743 ÷ n)². Multiply it by the bolt material's proof or tensile strength to get the load the thread can carry.
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