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1-3/8-6 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)
39/32 (1.2188")
31 mm · Clearance hole (normal fit): 1.5000" (1-1/2) ·
Grade 5 torque: 1469 lb-ft dry
Thread dimensions
Major diameter
1.375 "
Threads per inch
6
Pitch
4.233 mm
Pitch diameter
1.2668 "
Minor diameter
1.1946 "
Tensile stress area
1.15488 in²
Tap drill sizes for 1-3/8-6 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% | 1.2668" | 81/64 (1.2656") | 32 mm |
| 65% | 1.2343" | 79/64 (1.2344") | 31.5 mm |
| 75% | 1.2126" | 39/32 (1.2188") | 31 mm |
| 85% | 1.1910" | 19/16 (1.1875") | 30.5 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 1-3/8-6 UNC
| Fit | Drill | Use |
|---|---|---|
| Close | 1.4380" (1-7/16) | Located joints, dowel-like alignment |
| Normal | 1.5000" (1-1/2) | General assembly — use this unless you have a reason not to |
| Loose | 1.6090" (1-39/64) | Weldments, sheet stacks, tolerance build-up |
1-3/8-6 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 | 655 lb-ft | 491 lb-ft | 888 N·m | 28,583 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 1469 lb-ft | 1102 lb-ft | 1992 N·m | 64,096 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 2382 lb-ft | 1786 lb-ft | 3229 N·m | 103,939 lb |
| 18-8 SS 304/18-8 stainless | 794 lb-ft | 595 lb-ft | 1076 N·m | 34,646 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
2-1/16"
Hex key (socket head)
1