The tap drill for a 5/8-18 UNF thread is a 37/64" drill (14.68 mm), which leaves about 65% of full thread with a cutting tap. The ideal hole for 75% thread is 0.5709 in, and 37/64" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.5889 in | 19/32" | 0.5938 in / 15.08 mm | 43% |
| 60% | 0.5817 in | 37/64" | 0.5781 in / 14.68 mm | 65% |
| 65% | 0.5781 in | 37/64" | 0.5781 in / 14.68 mm | 65% |
| 70% | 0.5745 in | 37/64" | 0.5781 in / 14.68 mm | 65% |
| 75% | 0.5709 in | 37/64" | 0.5781 in / 14.68 mm | 65% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 22 points of thread apart. 9/16" (0.5625 in) gives 87% and a stronger but harder-to-tap thread; 37/64" (0.5781 in) gives 65% and less tapping torque. On 5/8-18 UNF the choice is mostly about tapping torque and machine power.
5/8-18 is used on automotive and truck parts, hydraulic fittings and some aircraft hardware. Its basic pitch diameter is 0.5889 in and its tensile stress area is 0.2560 in². A bolt of this size passes through a 0.6880 in normal-fit clearance hole (ASME B18.2.8).
Example: 5/8-18 UNF at 75% thread
Result: 37/64" (0.5781 in), 65% thread
| Major (nominal) diameter | 0.6250 in / 15.88 mm |
|---|---|
| Pitch | 18 TPI (0.0556 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.5889 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.5649 in |
| Tensile stress area | 0.2560 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 UNF sizes:
Use 37/64" (0.5781 in). It leaves 65% of full thread with a cutting tap; the ideal 75% hole is 0.5709 in. For an easier-to-tap 50% thread, 19/32" (0.5938 in) gives 43%.
37/64" is 14.68 mm. The nearest metric drill to the 75% target is 14.5 mm (0.5709 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.6004 in for 65% thread, so drill 19/32" (0.5938 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 5/8-18 UNF, 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 5/8-11 UNC uses the same sizes.
0.2560 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. 5/8-18 UNF takes 37/64" (0.5781 in); 5/8-11 UNC takes 17/32" (0.5313 in). Check the pitch on the drawing before you drill.
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