The tap drill for a 3/8-16 UNC thread is a 5/16" drill (7.94 mm), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 0.3141 in, and 5/16" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.3344 in | Q | 0.3320 in / 8.43 mm | 53% |
| 60% | 0.3263 in | 21/64" | 0.3281 in / 8.33 mm | 58% |
| 65% | 0.3222 in | P | 0.3230 in / 8.20 mm | 64% |
| 70% | 0.3182 in | O | 0.3160 in / 8.03 mm | 73% |
| 75% | 0.3141 in | 5/16" | 0.3125 in / 7.94 mm | 77% |
The drills either side of the target are 4 points of thread apart, so either is a reasonable choice. 5/16" (0.3125 in) gives 77% and a stronger but harder-to-tap thread; O (0.3160 in) gives 73% and less tapping torque. On 3/8-16 UNC the choice is mostly about tap life in hard materials.
3/8-16 fastens machine bases, vises, structural brackets and many fixture clamps. Its basic pitch diameter is 0.3344 in and its tensile stress area is 0.0775 in². A bolt of this size passes through a 0.4060 in normal-fit clearance hole (ASME B18.2.8).
Example: 3/8-16 UNC at 75% thread
Result: 5/16" (0.3125 in), 77% thread
| Major (nominal) diameter | 0.3750 in / 9.52 mm |
|---|---|
| Pitch | 16 TPI (0.0625 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.3344 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.3073 in |
| Tensile stress area | 0.0775 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.3910 in / 9.93 mm |
| Normal | 0.4060 in / 10.31 mm |
| Loose | 0.4220 in / 10.72 mm |
All UNC sizes:
Use 5/16" (0.3125 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 0.3141 in. For an easier-to-tap 50% thread, Q (0.3320 in) gives 53%.
5/16" is 7.94 mm. The nearest metric drill to the 75% target is 8 mm (0.3150 in), which gives 74% thread.
A forming tap needs a bigger hole: 0.3474 in for 65% thread, so drill S (0.3480 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 3/8-16 UNC, ASME B18.2.8 gives close fit 0.3910 in, normal fit 0.4060 in and loose fit 0.4220 in. The hole depends only on the diameter, so 3/8-24 UNF uses the same sizes.
0.0775 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. 3/8-16 UNC takes 5/16" (0.3125 in); 3/8-24 UNF takes Q (0.3320 in). Check the pitch on the drawing before you drill.
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