The tap drill for a 5/16-18 UNC thread is a F drill (6.53 mm), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 0.2584 in, and F is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.2764 in | J | 0.2770 in / 7.04 mm | 49% |
| 60% | 0.2692 in | I | 0.2720 in / 6.91 mm | 56% |
| 65% | 0.2656 in | 17/64" | 0.2656 in / 6.75 mm | 65% |
| 70% | 0.2620 in | G | 0.2610 in / 6.63 mm | 71% |
| 75% | 0.2584 in | F | 0.2570 in / 6.53 mm | 77% |
The drills either side of the target are 6 points of thread apart, so either is a reasonable choice. F (0.2570 in) gives 77% and a stronger but harder-to-tap thread; G (0.2610 in) gives 71% and less tapping torque. On 5/16-18 UNC the choice is mostly about tap life in hard materials.
5/16-18 is used on automotive brackets, small engine parts, trailer hardware and machine frames. Its basic pitch diameter is 0.2764 in and its tensile stress area is 0.0524 in². A bolt of this size passes through a 0.3440 in normal-fit clearance hole (ASME B18.2.8).
Example: 5/16-18 UNC at 75% thread
Result: F (0.2570 in), 77% thread
| Major (nominal) diameter | 0.3125 in / 7.94 mm |
|---|---|
| Pitch | 18 TPI (0.0556 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.2764 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.2524 in |
| Tensile stress area | 0.0524 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.3280 in / 8.33 mm |
| Normal | 0.3440 in / 8.74 mm |
| Loose | 0.3590 in / 9.12 mm |
All UNC sizes:
Use F (0.2570 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 0.2584 in. For an easier-to-tap 50% thread, J (0.2770 in) gives 49%.
F is 6.53 mm. The nearest metric drill to the 75% target is 6.6 mm (0.2598 in), which gives 73% thread.
A forming tap needs a bigger hole: 0.2879 in for 65% thread, so drill L (0.2900 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 5/16-18 UNC, ASME B18.2.8 gives close fit 0.3280 in, normal fit 0.3440 in and loose fit 0.3590 in. The hole depends only on the diameter, so 5/16-24 UNF uses the same sizes.
0.0524 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. 5/16-18 UNC takes F (0.2570 in); 5/16-24 UNF takes I (0.2720 in). Check the pitch on the drawing before you drill.
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