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1-1/2-6 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)
43/32 (1.3438")
34 mm · Clearance hole (normal fit): 1.6250" (1-5/8) ·
Grade 5 torque: 1950 lb-ft dry
Thread dimensions
Major diameter
1.5 "
Threads per inch
6
Pitch
4.233 mm
Pitch diameter
1.3918 "
Minor diameter
1.3196 "
Tensile stress area
1.40525 in²
Tap drill sizes for 1-1/2-6 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% | 1.3918" | 89/64 (1.3906") | 35.5 mm |
| 65% | 1.3593" | 87/64 (1.3594") | 34.5 mm |
| 75% | 1.3376" | 43/32 (1.3438") | 34 mm |
| 85% | 1.3160" | 21/16 (1.3125") | 33.5 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 1-1/2-6 UNC
| Fit | Drill | Use |
|---|---|---|
| Close | 1.5620" (1-9/16) | Located joints, dowel-like alignment |
| Normal | 1.6250" (1-5/8) | General assembly — use this unless you have a reason not to |
| Loose | 1.7340" (1-47/64) | Weldments, sheet stacks, tolerance build-up |
1-1/2-6 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 | 869 lb-ft | 652 lb-ft | 1179 N·m | 34,779 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 1950 lb-ft | 1462 lb-ft | 2644 N·m | 77,991 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 3162 lb-ft | 2371 lb-ft | 4287 N·m | 126,472 lb |
| 18-8 SS 304/18-8 stainless | 1054 lb-ft | 790 lb-ft | 1429 N·m | 42,157 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
2-1/4"
Hex key (socket head)
1