The tap drill for a 1/2-13 UNC thread is a 27/64" drill (10.72 mm), which leaves about 78% of full thread with a cutting tap. The ideal hole for 75% thread is 0.4251 in, and 27/64" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.4500 in | 29/64" | 0.4531 in / 11.51 mm | 47% |
| 60% | 0.4400 in | 7/16" | 0.4375 in / 11.11 mm | 63% |
| 65% | 0.4350 in | 7/16" | 0.4375 in / 11.11 mm | 63% |
| 70% | 0.4301 in | 7/16" | 0.4375 in / 11.11 mm | 63% |
| 75% | 0.4251 in | 27/64" | 0.4219 in / 10.72 mm | 78% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 16 points of thread apart. 27/64" (0.4219 in) gives 78% and a stronger but harder-to-tap thread; 7/16" (0.4375 in) gives 63% and less tapping torque. On 1/2-13 UNC the choice is mostly about tap life in hard materials.
1/2-13 is a standard bolt for machine bases, structural steel, die sets and heavy fixtures. Its basic pitch diameter is 0.4500 in and its tensile stress area is 0.1419 in². A bolt of this size passes through a 0.5620 in normal-fit clearance hole (ASME B18.2.8).
Example: 1/2-13 UNC at 75% thread
Result: 27/64" (0.4219 in), 78% thread
| Major (nominal) diameter | 0.5000 in / 12.70 mm |
|---|---|
| Pitch | 13 TPI (0.0769 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.4500 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.4167 in |
| Tensile stress area | 0.1419 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.5310 in / 13.49 mm |
| Normal | 0.5620 in / 14.27 mm |
| Loose | 0.6090 in / 15.47 mm |
All UNC sizes:
Use 27/64" (0.4219 in). It leaves 78% of full thread with a cutting tap; the ideal 75% hole is 0.4251 in. For an easier-to-tap 50% thread, 29/64" (0.4531 in) gives 47%.
27/64" is 10.72 mm. The nearest metric drill to the 75% target is 10.8 mm (0.4252 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.4660 in for 65% thread, so drill 15/32" (0.4688 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 1/2-13 UNC, ASME B18.2.8 gives close fit 0.5310 in, normal fit 0.5620 in and loose fit 0.6090 in. The hole depends only on the diameter, so 1/2-20 UNF uses the same sizes.
0.1419 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. 1/2-13 UNC takes 27/64" (0.4219 in); 1/2-20 UNF takes 29/64" (0.4531 in). Check the pitch on the drawing before you drill.
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