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M8×1.25 — 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)
6.8 mm
Nearest inch drill: H (0.2660") · Clearance hole (normal fit): 9 mm ·
8.8 torque: 25.5 N·m dry
Thread dimensions
Major diameter
8 mm
Pitch
1.25 mm
TPI (equivalent)
20.32
Pitch diameter
7.188 mm
Minor diameter
6.647 mm
Tensile stress area
36.61 mm²
Tap drill sizes for M8×1.25
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% | 7.188 mm | 7.2 mm | 0.2812" (9/32 drill) |
| 65% | 6.945 mm | 6.9 mm | 0.2720" (I drill) |
| 75% | 6.782 mm | 6.8 mm | 0.2660" (H drill) |
| 85% | 6.62 mm | 6.6 mm | 0.2610" (G 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 M8×1.25
| Fit | Hole | Use |
|---|---|---|
| Close | 8.4 mm | Located joints, dowel-like alignment |
| Normal | 9 mm | General assembly — use this unless you have a reason not to |
| Loose | 10 mm | Weldments, sheet stacks, tolerance build-up |
M8×1.25 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 | 9.88 N·m | 7.41 N·m | 7.29 lb-ft | 6.2 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 25.5 N·m | 19.1 N·m | 18.8 lb-ft | 15.9 kN |
| 10.9 Alloy steel, Q&T — high strength | 36.5 N·m | 27.3 N·m | 26.9 lb-ft | 22.8 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 42.6 N·m | 32 N·m | 31.4 lb-ft | 26.6 kN |
| A2-70 304 stainless, cold worked | 19.8 N·m | 14.8 N·m | 14.6 lb-ft | 12.4 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
13 mm
Hex key (socket head)
6 mm