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5/8-18 UNF — Full Fastener Spec
ASME B1.1 (Unified) · Unf series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
37/64 (0.5781")
14.5 mm · Clearance hole (normal fit): 0.6880" (11/16) ·
Grade 5 torque: 170 lb-ft dry
Thread dimensions
Major diameter
0.625 "
Threads per inch
18
Pitch
1.411 mm
Pitch diameter
0.5889 "
Minor diameter
0.5649 "
Tensile stress area
0.25596 in²
Tap drill sizes for 5/8-18 UNF
Percent of thread is a trade-off: 75% is the shop default because the last 25% of engagement adds only a few percent of strength while roughly doubling tapping torque. Drop to 50–65% in tough or gummy material, or for blind holes where a snapped tap means scrapping the part.
| Thread engagement | Exact | Inch drill | Nearest metric drill |
|---|---|---|---|
| 50% | 0.5889" | 19/32 (0.5938") | 15 mm |
| 65% | 0.5781" | 37/64 (0.5781") | 14.7 mm |
| 75% | 0.5709" | 37/64 (0.5781") | 14.5 mm |
| 85% | 0.5637" | 9/16 (0.5625") | 14.3 mm |
Roll (form) taps need a larger hole than cut taps — add roughly
0.4× the pitch to the 75% cut-tap drill and confirm against your tap maker's chart.
Clearance hole for 5/8-18 UNF
| Fit | Drill | Use |
|---|---|---|
| Close | 0.6560" (21/32) | Located joints, dowel-like alignment |
| Normal | 0.6880" (11/16) | General assembly — use this unless you have a reason not to |
| Loose | 0.7340" (47/64) | Weldments, sheet stacks, tolerance build-up |
5/8-18 UNF torque specs
Calculated with the short-form equation T = K · D · F, using K = 0.2 (plain, dry, as-received steel) and a target clamp load of 75% of proof load — the standard preload target for a reusable joint.
| Grade | Dry | Lubricated | Dry (N·m) | Clamp load |
|---|---|---|---|---|
| Grade 2 Low/medium carbon steel, unmarked head | 110 lb-ft | 82.5 lb-ft | 149 N·m | 10,558 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 170 lb-ft | 127 lb-ft | 230 N·m | 16,317 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 240 lb-ft | 180 lb-ft | 325 N·m | 23,036 lb |
| 18-8 SS 304/18-8 stainless | 80 lb-ft | 60 lb-ft | 108 N·m | 7,678 lb |
The nut factor K dominates this result. Zinc-plated, waxed, anti-seize
and lubricated threads all shift K between roughly 0.10 and 0.25, which is a 2.5×
spread in torque for identical preload. If preload actually matters, torque-to-yield,
angle control or a load-indicating washer beats a torque wrench every time.
Tool sizes
Hex head wrench
15/16"
Hex key (socket head)
1/2