Bolt Grades Explained: Grade 2 vs Grade 5 vs Grade 8

Bolt Grades Explained

Understanding the difference between Grade 2, Grade 5, and Grade 8 bolts — and when to use each one.

Bolt grades indicate mechanical properties — primarily strength and hardness. Using the correct grade ensures your fasteners can handle the load without failing. Using the wrong grade can lead to equipment damage or safety hazards.

⚠️ Higher Grade ≠ Always Better

Harder bolts are more brittle. A Grade 8 bolt may crack under shock loads where a Grade 5 would flex and survive. Always match the grade to your application requirements.

US Bolt Grades

2

Grade 2

Low Carbon Steel

Tensile Strength
74,000 PSI
Proof Load
55,000 PSI
—
Head Marking: No marks (plain head)

The most basic and economical grade. Adequate for general purpose, non-critical applications where high strength isn't required.

Best For

  • General construction
  • Non-structural assemblies
  • Light-duty household projects
  • Cost-sensitive applications
5

Grade 5

Medium Carbon Steel, Quenched & Tempered

Tensile Strength
120,000 PSI
Proof Load
85,000 PSI
III
Head Marking: 3 radial lines (like a peace sign)

The workhorse grade for most automotive and industrial applications. Excellent balance of strength, ductility, and cost.

Best For

  • Automotive assemblies
  • Machinery and equipment
  • Structural connections
  • Most industrial applications
8

Grade 8

Medium Carbon Alloy Steel, Q&T

Tensile Strength
150,000 PSI
Proof Load
120,000 PSI
IIIIII
Head Marking: 6 radial lines

The strongest common grade. Use when maximum strength is required and the application is engineered for high-strength fasteners.

Best For

  • Suspension and steering
  • High-stress connections
  • Heavy equipment
  • Demanding industrial uses

Quick Comparison

Property Grade 2 Grade 5 Grade 8
Tensile Strength 74,000 PSI 120,000 PSI 150,000 PSI
Proof Load 55,000 PSI 85,000 PSI 120,000 PSI
Head Marking None 3 lines 6 lines
Material Low carbon steel Medium carbon, Q&T Alloy steel, Q&T
Relative Cost $ $$ $$$
Ductility High (softer) Medium Lower (harder)

💡 Quick Rule for Head Markings

Count the lines and add 2. A bolt with 3 lines is Grade 5 (3+2=5). A bolt with 6 lines is Grade 8 (6+2=8). No lines means Grade 2 or lower.

When to Use Each Grade

Grade 2: General Purpose

Use for non-critical applications: furniture assembly, fencing, light brackets, general household repairs. If the fastener fails, no one gets hurt and nothing expensive breaks.

Grade 5: Industrial Standard

The default choice for automotive, machinery, and most industrial work. Provides excellent strength for the price. Use when you need reliability but don't need maximum strength.

Grade 8: High Stress Only

Reserve for applications specifically designed for Grade 8: suspension components, steering linkages, heavy machinery, demanding industrial uses. Don't use "just to be safe" — the extra hardness means less ductility.

⚠️ Never Substitute Grades Without Engineering Review

Replacing a Grade 5 bolt with Grade 8 can be dangerous. The joint may have been designed for the Grade 5's flexibility. Similarly, never substitute a lower grade where a higher grade is specified.

Shop Bolts by Grade

We stock hex bolts, cap screws, and more in Grade 2, 5, and 8. Same-day shipping on orders by 2pm.

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