The tap drill for a #4-40 UNC thread is a #43 drill (2.26 mm), which leaves about 71% of full thread with a cutting tap. The ideal hole for 75% thread is 0.0876 in, and #43 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.0958 in | #41 | 0.0960 in / 2.44 mm | 49% |
| 60% | 0.0925 in | #42 | 0.0935 in / 2.37 mm | 57% |
| 65% | 0.0909 in | #43 | 0.0890 in / 2.26 mm | 71% |
| 70% | 0.0893 in | #43 | 0.0890 in / 2.26 mm | 71% |
| 75% | 0.0876 in | #43 | 0.0890 in / 2.26 mm | 71% |
The drills either side of the target are 9 points of thread apart, so either is a reasonable choice. #44 (0.0860 in) gives 80% and a stronger but harder-to-tap thread; #43 (0.0890 in) gives 71% and less tapping torque. On #4-40 UNC the choice is mostly about tap breakage.
#4-40 is the classic screw for circuit board standoffs, connector jack screws and small brackets. Its basic pitch diameter is 0.0958 in and its tensile stress area is 0.0060 in². A bolt of this size passes through a 0.1280 in normal-fit clearance hole (ASME B18.2.8).
Example: #4-40 UNC at 75% thread
Result: #43 (0.0890 in), 71% thread
| Major (nominal) diameter | 0.1120 in / 2.84 mm |
|---|---|
| Pitch | 40 TPI (0.0250 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.0958 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.0849 in |
| Tensile stress area | 0.0060 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.1200 in / 3.05 mm |
| Normal | 0.1280 in / 3.25 mm |
| Loose | 0.1440 in / 3.66 mm |
All UNC sizes:
Use #43 (0.0890 in). It leaves 71% of full thread with a cutting tap; the ideal 75% hole is 0.0876 in. For an easier-to-tap 50% thread, #41 (0.0960 in) gives 49%.
#43 is 2.26 mm. The nearest metric drill to the 75% target is 2.25 mm (0.0886 in), which gives 72% thread.
A forming tap needs a bigger hole: 0.1009 in for 65% thread, so drill #38 (0.1015 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #4-40 UNC, ASME B18.2.8 gives close fit 0.1200 in, normal fit 0.1280 in and loose fit 0.1440 in. Normal fit suits most bolted joints; close fit locates parts more accurately.
0.0060 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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