The tap drill for a 9/16-12 UNC thread is a 31/64" drill (12.30 mm), which leaves about 72% of full thread with a cutting tap. The ideal hole for 75% thread is 0.4813 in, and 31/64" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.5084 in | 33/64" | 0.5156 in / 13.10 mm | 43% |
| 60% | 0.4975 in | 1/2" | 0.5000 in / 12.70 mm | 58% |
| 65% | 0.4921 in | 31/64" | 0.4844 in / 12.30 mm | 72% |
| 70% | 0.4867 in | 31/64" | 0.4844 in / 12.30 mm | 72% |
| 75% | 0.4813 in | 31/64" | 0.4844 in / 12.30 mm | 72% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 14 points of thread apart. 15/32" (0.4688 in) gives 87% and a stronger but harder-to-tap thread; 31/64" (0.4844 in) gives 72% and less tapping torque. On 9/16-12 UNC the choice is mostly about tap life in hard materials.
9/16-12 shows up on heavy equipment, agricultural implements and some truck hardware. Its basic pitch diameter is 0.5084 in and its tensile stress area is 0.1819 in². ASME B18.2.8 lists no clearance hole for this diameter.
Example: 9/16-12 UNC at 75% thread
Result: 31/64" (0.4844 in), 72% thread
| Major (nominal) diameter | 0.5625 in / 14.29 mm |
|---|---|
| Pitch | 12 TPI (0.0833 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.5084 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.4723 in |
| Tensile stress area | 0.1819 in² |
ASME B18.2.8 does not list a clearance hole for this diameter. Take it from the bolt maker or your drawing standard.
All UNC sizes:
Use 31/64" (0.4844 in). It leaves 72% of full thread with a cutting tap; the ideal 75% hole is 0.4813 in. For an easier-to-tap 50% thread, 33/64" (0.5156 in) gives 43%.
31/64" is 12.30 mm. The nearest metric drill to the 75% target is 12 mm (0.4724 in), which gives 83% thread.
A forming tap needs a bigger hole: 0.5257 in for 65% thread, so drill 17/32" (0.5313 in). Forming taps work in ductile metals such as aluminum and mild steel.
ASME B18.2.8 does not list a clearance hole for 9/16" bolts. Use the bolt maker's recommendation or your company drawing standard.
0.1819 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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