The tap drill for a 3/4-10 UNC thread is a 21/32" drill (16.67 mm), which leaves about 72% of full thread with a cutting tap. The ideal hole for 75% thread is 0.6526 in, and 21/32" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.6850 in | 11/16" | 0.6875 in / 17.46 mm | 48% |
| 60% | 0.6721 in | 43/64" | 0.6719 in / 17.07 mm | 60% |
| 65% | 0.6656 in | 43/64" | 0.6719 in / 17.07 mm | 60% |
| 70% | 0.6591 in | 21/32" | 0.6563 in / 16.67 mm | 72% |
| 75% | 0.6526 in | 21/32" | 0.6563 in / 16.67 mm | 72% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 12 points of thread apart. 41/64" (0.6406 in) gives 84% and a stronger but harder-to-tap thread; 21/32" (0.6563 in) gives 72% and less tapping torque. On 3/4-10 UNC the choice is mostly about tapping torque and machine power.
3/4-10 fastens heavy machinery, structural steel, large flanges and lifting lugs. Its basic pitch diameter is 0.6850 in and its tensile stress area is 0.3345 in². A bolt of this size passes through a 0.8120 in normal-fit clearance hole (ASME B18.2.8).
Example: 3/4-10 UNC at 75% thread
Result: 21/32" (0.6563 in), 72% thread
| Major (nominal) diameter | 0.7500 in / 19.05 mm |
|---|---|
| Pitch | 10 TPI (0.1000 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.6850 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.6417 in |
| Tensile stress area | 0.3345 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.7810 in / 19.84 mm |
| Normal | 0.8120 in / 20.62 mm |
| Loose | 0.9060 in / 23.01 mm |
All UNC sizes:
Use 21/32" (0.6563 in). It leaves 72% of full thread with a cutting tap; the ideal 75% hole is 0.6526 in. For an easier-to-tap 50% thread, 11/16" (0.6875 in) gives 48%.
21/32" is 16.67 mm. The nearest metric drill to the 75% target is 16.5 mm (0.6496 in), which gives 77% thread.
A forming tap needs a bigger hole: 0.7058 in for 65% thread, so drill 45/64" (0.7031 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 3/4-10 UNC, ASME B18.2.8 gives close fit 0.7810 in, normal fit 0.8120 in and loose fit 0.9060 in. The hole depends only on the diameter, so 3/4-16 UNF uses the same sizes.
0.3345 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. 3/4-10 UNC takes 21/32" (0.6563 in); 3/4-16 UNF takes 11/16" (0.6875 in). Check the pitch on the drawing before you drill.
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