Home › 9/16-18 UNF
9/16-18 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)
33/64 (0.5156")
12.9 mm · Clearance hole (normal fit): 0.6250" (5/8) ·
Grade 5 torque: 121 lb-ft dry
Thread dimensions
Major diameter
0.5625 "
Threads per inch
18
Pitch
1.411 mm
Pitch diameter
0.5264 "
Minor diameter
0.5024 "
Tensile stress area
0.20298 in²
Tap drill sizes for 9/16-18 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% | 0.5264" | 17/32 (0.5312") | 13.4 mm |
| 65% | 0.5156" | 33/64 (0.5156") | 13.1 mm |
| 75% | 0.5084" | 33/64 (0.5156") | 12.9 mm |
| 85% | 0.5012" | 1/2 (0.5000") | 12.7 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 9/16-18 UNF
| Fit | Drill | Use |
|---|---|---|
| Close | 0.5940" (19/32) | Located joints, dowel-like alignment |
| Normal | 0.6250" (5/8) | General assembly — use this unless you have a reason not to |
| Loose | 0.6720" (43/64) | Weldments, sheet stacks, tolerance build-up |
9/16-18 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 | 78.5 lb-ft | 58.9 lb-ft | 106 N·m | 8,372 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 121 lb-ft | 91 lb-ft | 164 N·m | 12,940 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 171 lb-ft | 128 lb-ft | 232 N·m | 18,268 lb |
| 18-8 SS 304/18-8 stainless | 57.1 lb-ft | 42.8 lb-ft | 77.4 N·m | 6,089 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
13/16"
Hex key (socket head)
7/16