5/8-11 Tap Drill Size
Unified coarse (UNC) · 11 threads per inch · ASME B1.1, class 2B
For a 5/8 tap: 5/8-11 coarse takes a 17/32" drill, 5/8-18 fine takes a 37/64" drill.
What size drill bit for a 5/8-11 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 17/32"Use | 0.5313 | 13.49 | ✓ | |
| 13.5 mm | 0.5315 | 13.50 | ✓ | |
| 35/64" | 0.5469 | 13.89 | ✗ | |
| 14 mm | 0.5512 | 14.00 | ✗ |
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 (0.527 in to 0.546 in).
5/8-11 UNC ASME B1.1, class 2B
| Major diameter | 0.6250 in / 15.88 mm |
|---|---|
| Threads per inch | 11 TPI (pitch 0.0909 in) |
| Pitch diameter (basic) | 0.5660 in |
| Minor diameter 2B | 0.5266–0.5460 in |
| Thread-forming tap drill | 37/64" 0.5781 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
- 0.625 inthread length for full strength in steel (1 × D)
- 0.938 inin cast iron or brass (1.5 × D), up to 1.250 in in aluminum
- +0.455 inextra drill depth below the thread for a plug tap (5 pitches), +0.182 in with a bottoming tap
Shop notes
5/8-11 has almost no slack. The standard 17/32 drill (0.5313 in) gives about 79% thread, a 13.5 mm drill makes practically the same hole, and that is the whole list of good choices. The next fractional size, 35/64, drops to about 66% and is already past the maximum minor diameter. A 14 mm is worse at 62%, and a 9/16 is too large to even make the table.
So the advice is short: get the 17/32 or the 13.5 mm and do not improvise. If 79% feels heavy in hard steel, the answer is a sharp tap, cutting oil and a wrench long enough to turn it steadily, not a bigger drill.
You will meet 5/8-11 on the spindle of most angle grinders sold in North America, where it holds the flange nut and threaded cup wheels.
Not an M16
M16 x 2 is 0.630 in across, a hair over 5/8 (0.625 in), and the pitches are close: 2 mm against 2.31 mm. Close is not compatible, and a bolt of one will start in the other and then lock up. The drills tell the same story: the 14 mm that is right for M16 x 2 leaves a 5/8-11 hole at 62% and out of spec.