5/8″-18 UNF Thread
The chart recommendation gives approximately 65% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.
Specification
| Designation | 5/8″-18 UNF |
|---|---|
| Thread family | Unified National Fine (UNF) |
| Nominal size | 5/8 |
| Basic major diameter | 0.6250″ / 15.875 mm |
| Threads per inch / pitch | 18 TPI (1.411 mm pitch) |
| Recommended tap drill | 37/64″ (0.5781″ / 14.684 mm) |
Tap drill options by thread engagement
Any of these standard drills produces a usable engagement for 5/8″-18 UNF; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).
| Drill | Decimal in | mm | Approx. engagement |
|---|---|---|---|
| 14.5 mm | 0.5709 | 14.5 | 75% |
| 37/64″ — chart recommendation | 0.5781 | 14.684 | 65% |
Nearest drills to the tap drill in other systems
| Drill | Size | vs 37/64″ | Resulting engagement |
|---|---|---|---|
| 14.5 mm | 0.5709″ / 14.5 mm | −0.184 mm | 75% |
Clearance holes
For a bolt or screw of this size to pass through freely. Recommended drills per ASME B18.2.8: close, normal, and loose fits.
| Fit | Drill / Ø | Decimal in | mm |
|---|---|---|---|
| close | 21/32″ | 0.6563 | 16.669 |
| normal | 11/16″ | 0.6875 | 17.462 |
| loose | 47/64″ | 0.7344 | 18.653 |
Hex bolt & nut dimensions
For a standard hex cap screw (ASME B18.2.1) and finished hex nut (ASME B18.2.2) in this size.
| Wrench / socket size (bolt head) | 15/16″ |
|---|---|
| Head height | 25/64″ (0.3906″) |
| Nut thickness | 35/64″ (0.5469″) |
| Compatible nut | 5/8-18 hex nut |
Basic (nominal) dimensions. See the wrench & socket size charts for all sizes and the metric ↔ SAE socket comparison.
Related threads
- 5/8″-11 UNC — same nominal size, 11 TPI (Unified National Coarse (UNC)) · compare
- 9/16″-18 UNF — next size down in UNF
- 3/4″-16 UNF — next size up in UNF