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#3-56 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)
#45 (0.0820")
2.1 mm · Clearance hole (normal fit): 0.1160" (#32) ·
Grade 5 torque: 0.55 lb-ft dry
Thread dimensions
Major diameter
0.099 "
Threads per inch
56
Pitch
0.454 mm
Pitch diameter
0.0874 "
Minor diameter
0.0797 "
Tensile stress area
0.00523 in²
Tap drill sizes for #3-56 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% | 0.0874" | #44 (0.0860") | 2.2 mm |
| 65% | 0.0839" | #45 (0.0820") | 2.1 mm |
| 75% | 0.0816" | #45 (0.0820") | 2.1 mm |
| 85% | 0.0793" | #47 (0.0785") | 2 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 #3-56 UNF
| Fit | Drill | Use |
|---|---|---|
| Close | 0.1060" (#36) | Located joints, dowel-like alignment |
| Normal | 0.1160" (#32) | General assembly — use this unless you have a reason not to |
| Loose | 0.1280" (#30) | Weldments, sheet stacks, tolerance build-up |
#3-56 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 | 0.356 lb-ft | 0.267 lb-ft | 0.483 N·m | 215 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 0.55 lb-ft | 0.413 lb-ft | 0.746 N·m | 333 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 0.777 lb-ft | 0.583 lb-ft | 1.05 N·m | 470 lb |
| 18-8 SS 304/18-8 stainless | 0.259 lb-ft | 0.194 lb-ft | 0.351 N·m | 156 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
3/16"
Hex key (socket head)
5/64