Torque Specifications for Construction Bolts: Complete Reference Guide

Correct bolt torque is one of the most misunderstood parts of fastener installation on construction sites. Too loose and the joint fails over time. Too tight and you risk thread damage, bolt failure, or crushing the materials you’re fixing.

This guide explains bolt torque specifications in plain terms — what torque actually does, how to apply it correctly, and what matters in real construction environments (not just lab conditions).

What Torque Really Does in a Bolted Joint

Torque is not about “tightness” — it’s about clamp force.

When you torque a bolt, you’re stretching it slightly. That stretch creates clamping force, which holds the joint together. The bolt isn’t meant to carry the load by itself — the friction between the joined materials does most of the work.

That’s why correct torque requirements are critical to proper fastener installation.

Why Bolt Torque Specs Matter on Construction Sites

On Australian construction sites, torque problems usually come from:

• Guesswork with impact guns
• Dirty or damaged threads
• Mixing bolt grades, nuts, and washers
• Ignoring washer requirements
• Over-tightening “to be safe”

Any of these can cause joint failure, even if the bolt itself is high strength.

Typical Bolt Torque Specs (General Reference Only)

Below is a general reference for common metric bolts. Always follow project specs and engineering documentation where provided.

Example – Metric Coarse Thread (Dry, Unlubricated):

• M10 Grade 8.8 → ~49 Nm
• M12 Grade 8.8 → ~85 Nm
• M16 Grade 8.8 → ~210 Nm
• M20 Grade 8.8 → ~410 Nm

Important: these are indicative only. Actual torque requirements vary based on:

• Bolt grade (8.8 vs 10.9)
• Thread condition
• Lubrication
• Washer type
• Joint material

The Biggest Torque Mistake: Lubrication

Lubrication dramatically changes torque results.

A lubricated bolt can generate 20–30% more clamp force at the same torque value compared to a dry bolt. That means if you torque a lubricated bolt to a “dry” spec, you may already be over-stressing it.

If lubrication is present:

• Use torque specs intended for lubricated fasteners
• Or reduce torque accordingly

This is one of the most common causes of snapped bolts during installation.

Impact Guns vs Torque Wrenches

Impact guns are great for running bolts down — they are not torque control tools.

For structural or critical connections:

• Run down with an impact
• Final tighten with a calibrated torque wrench
• Use the correct washer stack

Relying on “feel” might work on small jobs, but it’s not repeatable and it’s not defensible if something fails.

Bolt Grade and Torque Go Hand in Hand

Higher-strength bolts require higher torque to achieve the correct clamp force.

If you’re unsure how grades affect torque, read:
👉 Fastener Grades Explained: 4.6, 8.8, 10.9, and 12.9

Never substitute a higher-grade bolt without checking torque requirements — more strength does not mean “tighten harder”.

Torque Is Only One Part of Installation

Correct fastener installation also depends on:

• Clean threads
• Correct nut grade
• Proper washers
• Square joint faces
• Correct embedment (for anchors)

Torque alone won’t fix poor joint design.

Final Word on Torque Specifications

Torque specs are a tool, not a guess. Used correctly, they ensure consistency, safety, and long-term performance. Used incorrectly, they give a false sense of security.

If a connection matters, torque it properly — and document it if required.

👉 For structural applications, also see:
AS/NZS 1252 Compliance for Structural Bolts