3/4-16 Tap Drill Size
Unified fine (UNF) · 16 threads per inch · ASME B1.1, class 2B
For a 3/4 tap: 3/4-10 coarse takes a 21/32" drill, 3/4-16 fine takes an 11/16" drill.
What size drill bit for a 3/4-16 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 11/16"Use | 0.6875 | 17.46 | ✓ | |
| 17.5 mm | 0.6890 | 17.50 | ✓ | |
| 45/64" | 0.7031 | 17.86 | ✗ |
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.682 in to 0.696 in).
3/4-16 UNF ASME B1.1, class 2B
| Major diameter | 0.7500 in / 19.05 mm |
|---|---|
| Threads per inch | 16 TPI (pitch 0.0625 in) |
| Pitch diameter (basic) | 0.7094 in |
| Minor diameter 2B | 0.6823–0.6964 in |
| Thread-forming tap drill | 23/32" 0.7188 in |
Clearance hole ASME B18.2.8
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 0.750 inthread length for full strength in steel (1 × D)
- 1.125 inin cast iron or brass (1.5 × D), up to 1.500 in in aluminum
- +0.313 inextra drill depth below the thread for a plug tap (5 pitches), +0.125 in with a bottoming tap
Shop notes
An 11/16 drill gives about 77% thread and is the right answer, with a 17.5 mm drill as the metric twin at about 75%. Both are inside the 2B minor diameter limits. That is the whole list: the next fraction, 45/64, is out at about 58%, so 3/4-16 does not leave room for a "loose" substitute the way smaller fine threads do.
At this size the drill is rarely the problem. The tap is. A 3/4 fine tap in steel needs a long wrench, cutting oil and a hole drilled straight, the same as the coarse 3/4-10. Starting the tap square, with a guide if you have one, saves more threads than any choice of drill.
In hydraulics, 3/4-16 is the -8 size for SAE straight-thread O-ring ports and 37 degree flare (JIC) fittings. As with any port, the spot face and O-ring seat matter as much as the thread, so a port tool is the better route for a real hydraulic connection.
The coarse 3/4-10 drill, 21/32, is not a near miss: at 0.6562 in it is far under the 0.6823 in minimum minor diameter for 3/4-16.