The tap drill for a 7/16-14 UNC thread is a U drill (9.35 mm), which leaves about 75% of full thread with a cutting tap. The ideal hole for 75% thread is 0.3679 in, and U is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.3911 in | 25/64" | 0.3906 in / 9.92 mm | 51% |
| 60% | 0.3818 in | W | 0.3860 in / 9.80 mm | 56% |
| 65% | 0.3772 in | V | 0.3770 in / 9.58 mm | 65% |
| 70% | 0.3725 in | 3/8" | 0.3750 in / 9.52 mm | 67% |
| 75% | 0.3679 in | U | 0.3680 in / 9.35 mm | 75% |
The drills either side of the target are 9 points of thread apart, so either is a reasonable choice. 23/64" (0.3594 in) gives 84% and a stronger but harder-to-tap thread; U (0.3680 in) gives 75% and less tapping torque. On 7/16-14 UNC the choice is mostly about tap life in hard materials.
7/16-14 appears on automotive parts, some heavy equipment and older machine tools; it is less common than 3/8 or 1/2. Its basic pitch diameter is 0.3911 in and its tensile stress area is 0.1063 in². A bolt of this size passes through a 0.4690 in normal-fit clearance hole (ASME B18.2.8).
Example: 7/16-14 UNC at 75% thread
Result: U (0.3680 in), 75% thread
| Major (nominal) diameter | 0.4375 in / 11.11 mm |
|---|---|
| Pitch | 14 TPI (0.0714 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.3911 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.3602 in |
| Tensile stress area | 0.1063 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.4530 in / 11.51 mm |
| Normal | 0.4690 in / 11.91 mm |
| Loose | 0.4840 in / 12.29 mm |
All UNC sizes:
Use U (0.3680 in). It leaves 75% of full thread with a cutting tap; the ideal 75% hole is 0.3679 in. For an easier-to-tap 50% thread, 25/64" (0.3906 in) gives 51%.
U is 9.35 mm. The nearest metric drill to the 75% target is 9.3 mm (0.3661 in), which gives 77% thread.
A forming tap needs a bigger hole: 0.4059 in for 65% thread, so drill 13/32" (0.4063 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 7/16-14 UNC, ASME B18.2.8 gives close fit 0.4530 in, normal fit 0.4690 in and loose fit 0.4840 in. The hole depends only on the diameter, so 7/16-20 UNF uses the same sizes.
0.1063 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. 7/16-14 UNC takes U (0.3680 in); 7/16-20 UNF takes 25/64" (0.3906 in). Check the pitch on the drawing before you drill.
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