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M18×2.5 — Full Fastener Spec
ISO 68-1 / ISO 261 · Coarse series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
15.5 mm
Nearest inch drill: 39/64 (0.6094") · Clearance hole (normal fit): 20 mm ·
8.8 torque: 301 N·m dry
Thread dimensions
Major diameter
18 mm
Pitch
2.5 mm
TPI (equivalent)
10.16
Pitch diameter
16.376 mm
Minor diameter
15.294 mm
Tensile stress area
192.47 mm²
Tap drill sizes for M18×2.5
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 | Metric drill | Nearest inch drill |
|---|---|---|---|
| 50% | 16.376 mm | 16.4 mm | 0.6406" (41/64 drill) |
| 65% | 15.889 mm | 15.9 mm | 0.6250" (5/8 drill) |
| 75% | 15.564 mm | 15.5 mm | 0.6094" (39/64 drill) |
| 85% | 15.24 mm | 15.2 mm | 0.5938" (19/32 drill) |
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 M18×2.5
| Fit | Hole | Use |
|---|---|---|
| Close | 19 mm | Located joints, dowel-like alignment |
| Normal | 20 mm | General assembly — use this unless you have a reason not to |
| Loose | 21 mm | Weldments, sheet stacks, tolerance build-up |
M18×2.5 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 (lb-ft) | Clamp load |
|---|---|---|---|---|
| 4.6 Low-carbon steel, general purpose | 117 N·m | 87.7 N·m | 86.2 lb-ft | 32.5 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 301 N·m | 226 N·m | 222 lb-ft | 83.7 kN |
| 10.9 Alloy steel, Q&T — high strength | 431 N·m | 323 N·m | 318 lb-ft | 119.8 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 504 N·m | 378 N·m | 372 lb-ft | 140 kN |
| A2-70 304 stainless, cold worked | 234 N·m | 175 N·m | 172 lb-ft | 65 kN |
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
27 mm
Hex key (socket head)
14 mm