The tap drill for a 5/8-11 UNC thread is a 17/32" drill (13.49 mm), which leaves about 79% of full thread with a cutting tap. The ideal hole for 75% thread is 0.5364 in, and 17/32" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.5660 in | 9/16" | 0.5625 in / 14.29 mm | 53% |
| 60% | 0.5541 in | 35/64" | 0.5469 in / 13.89 mm | 66% |
| 65% | 0.5482 in | 35/64" | 0.5469 in / 13.89 mm | 66% |
| 70% | 0.5423 in | 35/64" | 0.5469 in / 13.89 mm | 66% |
| 75% | 0.5364 in | 17/32" | 0.5313 in / 13.49 mm | 79% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 13 points of thread apart. 17/32" (0.5313 in) gives 79% and a stronger but harder-to-tap thread; 35/64" (0.5469 in) gives 66% and less tapping torque. On 5/8-11 UNC the choice is mostly about tapping torque and machine power.
5/8-11 is used for machinery mounts, structural connections, anchor bolts and large fixture clamps. Its basic pitch diameter is 0.5660 in and its tensile stress area is 0.2260 in². A bolt of this size passes through a 0.6880 in normal-fit clearance hole (ASME B18.2.8).
Example: 5/8-11 UNC at 75% thread
Result: 17/32" (0.5313 in), 79% thread
| Major (nominal) diameter | 0.6250 in / 15.88 mm |
|---|---|
| Pitch | 11 TPI (0.0909 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.5660 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.5266 in |
| Tensile stress area | 0.2260 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.6560 in / 16.66 mm |
| Normal | 0.6880 in / 17.48 mm |
| Loose | 0.7340 in / 18.64 mm |
All UNC sizes:
Use 17/32" (0.5313 in). It leaves 79% of full thread with a cutting tap; the ideal 75% hole is 0.5364 in. For an easier-to-tap 50% thread, 9/16" (0.5625 in) gives 53%.
17/32" is 13.49 mm. The nearest metric drill to the 75% target is 13.5 mm (0.5315 in), which gives 79% thread.
A forming tap needs a bigger hole: 0.5848 in for 65% thread, so drill 37/64" (0.5781 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 5/8-11 UNC, ASME B18.2.8 gives close fit 0.6560 in, normal fit 0.6880 in and loose fit 0.7340 in. The hole depends only on the diameter, so 5/8-18 UNF uses the same sizes.
0.2260 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. 5/8-11 UNC takes 17/32" (0.5313 in); 5/8-18 UNF takes 37/64" (0.5781 in). Check the pitch on the drawing before you drill.
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