The tap drill for a #10-24 UNC thread is a #25 drill (3.80 mm), which leaves about 75% of full thread with a cutting tap. The ideal hole for 75% thread is 0.1494 in, and #25 is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.1629 in | #20 | 0.1610 in / 4.09 mm | 54% |
| 60% | 0.1575 in | #22 | 0.1570 in / 3.99 mm | 61% |
| 65% | 0.1548 in | #23 | 0.1540 in / 3.91 mm | 67% |
| 70% | 0.1521 in | #24 | 0.1520 in / 3.86 mm | 70% |
| 75% | 0.1494 in | #25 | 0.1495 in / 3.80 mm | 75% |
The drills either side of the target are 5 points of thread apart, so either is a reasonable choice. #26 (0.1470 in) gives 79% and a stronger but harder-to-tap thread; #25 (0.1495 in) gives 75% and less tapping torque. On #10-24 UNC the choice is mostly about tap life in hard materials.
#10-24 is used for wood and sheet-metal assemblies, rack hardware and general machine screws where a coarse thread is wanted. Its basic pitch diameter is 0.1629 in and its tensile stress area is 0.0175 in². A bolt of this size passes through a 0.2210 in normal-fit clearance hole (ASME B18.2.8).
Example: #10-24 UNC at 75% thread
Result: #25 (0.1495 in), 75% thread
| Major (nominal) diameter | 0.1900 in / 4.83 mm |
|---|---|
| Pitch | 24 TPI (0.0417 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.1629 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.1449 in |
| Tensile stress area | 0.0175 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.2060 in / 5.23 mm |
| Normal | 0.2210 in / 5.61 mm |
| Loose | 0.2380 in / 6.05 mm |
All UNC sizes:
Use #25 (0.1495 in). It leaves 75% of full thread with a cutting tap; the ideal 75% hole is 0.1494 in. For an easier-to-tap 50% thread, #20 (0.1610 in) gives 54%.
#25 is 3.80 mm. The nearest metric drill to the 75% target is 3.8 mm (0.1496 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.1716 in for 65% thread, so drill 11/64" (0.1719 in). Forming taps work in ductile metals such as aluminum and mild steel.
For #10-24 UNC, ASME B18.2.8 gives close fit 0.2060 in, normal fit 0.2210 in and loose fit 0.2380 in. The hole depends only on the diameter, so #10-32 UNF uses the same sizes.
0.0175 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.
No. The drill depends on the pitch, not only the diameter. #10-24 UNC takes #25 (0.1495 in); #10-32 UNF takes #21 (0.1590 in). Check the pitch on the drawing before you drill.
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