The tap drill for a 7/16-20 UNF thread is a 25/64" drill (9.92 mm), which leaves about 72% of full thread with a cutting tap. The ideal hole for 75% thread is 0.3888 in, and 25/64" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.4050 in | Y | 0.4040 in / 10.26 mm | 52% |
| 60% | 0.3985 in | X | 0.3970 in / 10.08 mm | 62% |
| 65% | 0.3953 in | X | 0.3970 in / 10.08 mm | 62% |
| 70% | 0.3920 in | 25/64" | 0.3906 in / 9.92 mm | 72% |
| 75% | 0.3888 in | 25/64" | 0.3906 in / 9.92 mm | 72% |
The drills either side of the target are 7 points of thread apart, so either is a reasonable choice. W (0.3860 in) gives 79% and a stronger but harder-to-tap thread; 25/64" (0.3906 in) gives 72% and less tapping torque. On 7/16-20 UNF the choice is mostly about tap life in hard materials.
7/16-20 is used on hydraulic SAE O-ring boss ports (-4 size), automotive parts and aircraft hardware. Its basic pitch diameter is 0.4050 in and its tensile stress area is 0.1187 in². A bolt of this size passes through a 0.4690 in normal-fit clearance hole (ASME B18.2.8).
Example: 7/16-20 UNF at 75% thread
Result: 25/64" (0.3906 in), 72% thread
| Major (nominal) diameter | 0.4375 in / 11.11 mm |
|---|---|
| Pitch | 20 TPI (0.0500 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.4050 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.3834 in |
| Tensile stress area | 0.1187 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 UNF sizes:
Use 25/64" (0.3906 in). It leaves 72% of full thread with a cutting tap; the ideal 75% hole is 0.3888 in. For an easier-to-tap 50% thread, Y (0.4040 in) gives 52%.
25/64" is 9.92 mm. The nearest metric drill to the 75% target is 9.9 mm (0.3898 in), which gives 73% thread.
A forming tap needs a bigger hole: 0.4154 in for 65% thread, so drill Z (0.4130 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 7/16-20 UNF, 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 7/16-14 UNC uses the same sizes.
0.1187 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. 7/16-20 UNF takes 25/64" (0.3906 in); 7/16-14 UNC takes U (0.3680 in). Check the pitch on the drawing before you drill.
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