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#12-28 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)
#14 (0.1820")
4.6 mm · Clearance hole (normal fit): 0.2460" (D) ·
Grade 5 torque: 5.92 lb-ft dry
Thread dimensions
Major diameter
0.216 "
Threads per inch
28
Pitch
0.907 mm
Pitch diameter
0.1928 "
Minor diameter
0.1773 "
Tensile stress area
0.02579 in²
Tap drill sizes for #12-28 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% | 0.1928" | #10 (0.1935") | 4.9 mm |
| 65% | 0.1858" | #13 (0.1850") | 4.7 mm |
| 75% | 0.1812" | #14 (0.1820") | 4.6 mm |
| 85% | 0.1766" | #16 (0.1770") | 4.5 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 #12-28 UNF
| Fit | Drill | Use |
|---|---|---|
| Close | 0.2340" (A) | Located joints, dowel-like alignment |
| Normal | 0.2460" (D) | General assembly — use this unless you have a reason not to |
| Loose | 0.2656" (17/64) | Weldments, sheet stacks, tolerance build-up |
#12-28 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 | 3.83 lb-ft | 2.87 lb-ft | 5.19 N·m | 1,063 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 5.92 lb-ft | 4.44 lb-ft | 8.02 N·m | 1,644 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 8.36 lb-ft | 6.27 lb-ft | 11.3 N·m | 2,320 lb |
| 18-8 SS 304/18-8 stainless | 2.79 lb-ft | 2.09 lb-ft | 3.78 N·m | 773 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
7/16"
Hex key (socket head)
5/32