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M56×5.5 — 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)
50.5 mm
Nearest inch drill: 127/64 (1.9844") · Clearance hole (normal fit): 62 mm ·
8.8 torque: 9890 N·m dry
Thread dimensions
Major diameter
56 mm
Pitch
5.5 mm
TPI (equivalent)
4.62
Pitch diameter
52.428 mm
Minor diameter
50.046 mm
Tensile stress area
2030.02 mm²
Tap drill sizes for M56×5.5
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% | 52.428 mm | 52.5 mm | no standard inch drill |
| 65% | 51.356 mm | 51.5 mm | no standard inch drill |
| 75% | 50.641 mm | 50.5 mm | 1.9844" (127/64 drill) |
| 85% | 49.927 mm | 50 mm | 1.9688" (63/32 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 M56×5.5
| Fit | Hole | Use |
|---|---|---|
| Close | 58 mm | Located joints, dowel-like alignment |
| Normal | 62 mm | General assembly — use this unless you have a reason not to |
| Loose | 66 mm | Weldments, sheet stacks, tolerance build-up |
M56×5.5 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 | 3837 N·m | 2878 N·m | 2830 lb-ft | 342.6 kN |
| 8.8 Medium-carbon, quenched & tempered — the default structural grade | 9890 N·m | 7418 N·m | 7295 lb-ft | 883.1 kN |
| 10.9 Alloy steel, Q&T — high strength | 14153 N·m | 10615 N·m | 10439 lb-ft | 1263.7 kN |
| 12.9 Alloy steel, Q&T — highest common grade (socket head cap screws) | 16541 N·m | 12405 N·m | 12200 lb-ft | 1476.8 kN |
| A2-70 304 stainless, cold worked | 7673 N·m | 5755 N·m | 5660 lb-ft | 685.1 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
85 mm
Hex key (socket head)
41 mm