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#5-40 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)
#38 (0.1015")
2.6 mm · Clearance hole (normal fit): 0.1560" (5/32) ·
Grade 5 torque: 1.06 lb-ft dry
Thread dimensions
Major diameter
0.125 "
Threads per inch
40
Pitch
0.635 mm
Pitch diameter
0.1088 "
Minor diameter
0.0979 "
Tensile stress area
0.00796 in²
Tap drill sizes for #5-40 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% | 0.1088" | 7/64 (0.1094") | 2.8 mm |
| 65% | 0.1039" | #37 (0.1040") | 2.6 mm |
| 75% | 0.1006" | #38 (0.1015") | 2.6 mm |
| 85% | 0.0974" | #40 (0.0980") | 2.5 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 #5-40 UNC
| Fit | Drill | Use |
|---|---|---|
| Close | 0.1410" (9/64) | Located joints, dowel-like alignment |
| Normal | 0.1560" (5/32) | General assembly — use this unless you have a reason not to |
| Loose | 0.1720" (11/64) | Weldments, sheet stacks, tolerance build-up |
#5-40 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 | 0.684 lb-ft | 0.513 lb-ft | 0.927 N·m | 328 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 1.06 lb-ft | 0.792 lb-ft | 1.43 N·m | 507 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 1.49 lb-ft | 1.12 lb-ft | 2.02 N·m | 715 lb |
| 18-8 SS 304/18-8 stainless | 0.497 lb-ft | 0.373 lb-ft | 0.674 N·m | 238 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
5/16"
Hex key (socket head)
3/32