The tap drill for a 3/4-16 UNF thread is a 11/16" drill (17.46 mm), which leaves about 77% of full thread with a cutting tap. The ideal hole for 75% thread is 0.6891 in, and 11/16" is the nearest standard inch drill.
| Percent of thread | Ideal hole | Nearest drill | Drill size | Thread achieved |
|---|---|---|---|---|
| 50% | 0.7094 in | 45/64" | 0.7031 in / 17.86 mm | 58% |
| 60% | 0.7013 in | 45/64" | 0.7031 in / 17.86 mm | 58% |
| 65% | 0.6972 in | 45/64" | 0.7031 in / 17.86 mm | 58% |
| 70% | 0.6932 in | 11/16" | 0.6875 in / 17.46 mm | 77% |
| 75% | 0.6891 in | 11/16" | 0.6875 in / 17.46 mm | 77% |
Drill sizes are coarse relative to this pitch: the two drills either side of the target are 19 points of thread apart. 11/16" (0.6875 in) gives 77% and a stronger but harder-to-tap thread; 45/64" (0.7031 in) gives 58% and less tapping torque. On 3/4-16 UNF the choice is mostly about tapping torque and machine power.
3/4-16 is used on hydraulic -8 O-ring boss ports, axle nuts and large adjusters. Its basic pitch diameter is 0.7094 in and its tensile stress area is 0.3730 in². A bolt of this size passes through a 0.8120 in normal-fit clearance hole (ASME B18.2.8).
Example: 3/4-16 UNF at 75% thread
Result: 11/16" (0.6875 in), 77% thread
| Major (nominal) diameter | 0.7500 in / 19.05 mm |
|---|---|
| Pitch | 16 TPI (0.0625 in) |
| Basic pitch diameter (d − 0.6495·P) | 0.7094 in |
| Basic minor diameter, internal (d − 1.0825·P) | 0.6823 in |
| Tensile stress area | 0.3730 in² |
| Fit | Hole diameter |
|---|---|
| Close | 0.7810 in / 19.84 mm |
| Normal | 0.8120 in / 20.62 mm |
| Loose | 0.9060 in / 23.01 mm |
All UNF sizes:
Use 11/16" (0.6875 in). It leaves 77% of full thread with a cutting tap; the ideal 75% hole is 0.6891 in. For an easier-to-tap 50% thread, 45/64" (0.7031 in) gives 58%.
11/16" is 17.46 mm. The nearest metric drill to the 75% target is 17.5 mm (0.6890 in), which gives 75% thread.
A forming tap needs a bigger hole: 0.7224 in for 65% thread, so drill 23/32" (0.7188 in). Forming taps work in ductile metals such as aluminum and mild steel.
For 3/4-16 UNF, ASME B18.2.8 gives close fit 0.7810 in, normal fit 0.8120 in and loose fit 0.9060 in. The hole depends only on the diameter, so 3/4-10 UNC uses the same sizes.
0.3730 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. 3/4-16 UNF takes 11/16" (0.6875 in); 3/4-10 UNC takes 21/32" (0.6563 in). Check the pitch on the drawing before you drill.
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