The tap drill for a 3/8-24 UNF thread is a Q drill (8.43 mm), which leaves about 79% of full thread with a cutting tap. The ideal hole for 75% thread is 0.3344 in, and Q is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.3479 in | S | 0.3480 in / 8.84 mm | 50% |
| 60% | 0.3425 in | 11/32" | 0.3438 in / 8.73 mm | 58% |
| 65% | 0.3398 in | R | 0.3390 in / 8.61 mm | 67% |
| 70% | 0.3371 in | R | 0.3390 in / 8.61 mm | 67% |
| 75% | 0.3344 in | Q | 0.3320 in / 8.43 mm | 79% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 13 points of thread apart. Q (0.3320 in) gives 79% and a stronger but harder-to-tap thread; R (0.3390 in) gives 67% and less tapping torque. On 3/8-24 UNF the choice is mostly about tap life in hard materials.
3/8-24 is common on automotive suspension and steering parts, rod ends and aircraft fittings. Its basic pitch diameter is 0.3479 in and its tensile stress area is 0.0878 in². A bolt of this size passes through a 0.4060 in normal-fit clearance hole (ASME B18.2.8).
Example: 3/8-24 UNF at 75% thread
Result: Q (0.3320 in), 79% thread
| Major (nominal) diameter | 0.3750 in / 9.52 mm |
|---|---|
| Pitch | 24 TPI (0.0417 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.3479 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.3299 in |
| Tensile stress area | 0.0878 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 UNF sizes:
Use Q (0.3320 in). It leaves 79% of full thread with a cutting tap; the ideal 75% hole is 0.3344 in. For an easier-to-tap 50% thread, S (0.3480 in) gives 50%.
Q is 8.43 mm. The nearest metric drill to the 75% target is 8.5 mm (0.3346 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.3566 in for 65% thread, so drill T (0.3580 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 3/8-24 UNF, 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 3/8-16 UNC uses the same sizes.
0.0878 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. 3/8-24 UNF takes Q (0.3320 in); 3/8-16 UNC takes 5/16" (0.3125 in). Check the pitch on the drawing before you drill.
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