The tap drill for a 5/16-24 UNF thread is a I drill (6.91 mm), which leaves about 75% of full thread with a cutting tap. The ideal hole for 75% thread is 0.2719 in, and I is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.2854 in | 9/32" | 0.2813 in / 7.14 mm | 58% |
| 60% | 0.2800 in | K | 0.2810 in / 7.14 mm | 58% |
| 65% | 0.2773 in | J | 0.2770 in / 7.04 mm | 66% |
| 70% | 0.2746 in | J | 0.2770 in / 7.04 mm | 66% |
| 75% | 0.2719 in | I | 0.2720 in / 6.91 mm | 75% |
The drills either side of the target are 11 points of thread apart, so either is a reasonable choice. H (0.2660 in) gives 86% and a stronger but harder-to-tap thread; I (0.2720 in) gives 75% and less tapping torque. On 5/16-24 UNF the choice is mostly about tap life in hard materials.
5/16-24 appears on automotive connecting rods, brake parts and aircraft hardware where fine adjustment or vibration resistance matters. Its basic pitch diameter is 0.2854 in and its tensile stress area is 0.0581 in². A bolt of this size passes through a 0.3440 in normal-fit clearance hole (ASME B18.2.8).
Example: 5/16-24 UNF at 75% thread
Result: I (0.2720 in), 75% thread
| Major (nominal) diameter | 0.3125 in / 7.94 mm |
|---|---|
| Pitch | 24 TPI (0.0417 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.2854 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.2674 in |
| Tensile stress area | 0.0581 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 UNF sizes:
Use I (0.2720 in). It leaves 75% of full thread with a cutting tap; the ideal 75% hole is 0.2719 in. For an easier-to-tap 50% thread, 9/32" (0.2813 in) gives 58%.
I is 6.91 mm. The nearest metric drill to the 75% target is 6.9 mm (0.2717 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.2941 in for 65% thread, so drill M (0.2950 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 5/16-24 UNF, 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 5/16-18 UNC uses the same sizes.
0.0581 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. 5/16-24 UNF takes I (0.2720 in); 5/16-18 UNC takes F (0.2570 in). Check the pitch on the drawing before you drill.
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