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#2-56 UNC — Full Fastener Spec
ASME B1.1 (Unified) · Unc series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
#50 (0.0700")
1.7 mm · Clearance hole (normal fit): 0.1020" (#38) ·
Grade 5 torque: 0.338 lb-ft dry
Thread dimensions
Major diameter
0.086 "
Threads per inch
56
Pitch
0.454 mm
Pitch diameter
0.0744 "
Minor diameter
0.0667 "
Tensile stress area
0.0037 in²
Tap drill sizes for #2-56 UNC
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% | 0.0744" | #49 (0.0730") | 1.9 mm |
| 65% | 0.0709" | #50 (0.0700") | 1.8 mm |
| 75% | 0.0686" | #50 (0.0700") | 1.7 mm |
| 85% | 0.0663" | #51 (0.0670") | 1.7 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 #2-56 UNC
| Fit | Drill | Use |
|---|---|---|
| Close | 0.0940" (3/32) | Located joints, dowel-like alignment |
| Normal | 0.1020" (#38) | General assembly — use this unless you have a reason not to |
| Loose | 0.1160" (#32) | Weldments, sheet stacks, tolerance build-up |
#2-56 UNC 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 | 0.218 lb-ft | 0.164 lb-ft | 0.296 N·m | 152 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 0.338 lb-ft | 0.253 lb-ft | 0.458 N·m | 235 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 0.477 lb-ft | 0.358 lb-ft | 0.646 N·m | 332 lb |
| 18-8 SS 304/18-8 stainless | 0.159 lb-ft | 0.119 lb-ft | 0.215 N·m | 110 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