Understanding Bolt Grades: From Grade 2 to Grade 8 Explained

In manufacturing, construction, and industrial maintenance, not all bolts are created equal. One of the most important—and often overlooked—factors in choosing the right bolt is its grade. Bolt grade determines strength, durability, and performance under load. Using the wrong grade can lead to underperformance or catastrophic failure.

What Are Bolt Grades and Why Do They Matter?

A bolt grade refers to the mechanical properties of the bolt—mainly tensile strength, yield strength, and hardness. These grades are assigned by standards bodies like SAE (Society of Automotive Engineers) in the U.S.

Using the wrong bolt grade can result in:

• Stripped threads

• Bolt failure under tension

• Safety hazards

• Costly equipment damage

For example, a Grade 2 bolt used in a high-load machine application could shear or stretch permanently, while a Grade 8 bolt offers greater tensile strength and hardness.

Common SAE Bolt Grades

• None or “2”~74,000~57,000Low carbon steel

• Three lines~120,000~92,000Medium carbon steel, quenched and tempered

• Sixlines~150,000~130,000Medium carbon alloy steel, quenched and tempered

Note: Tensile strength = the maximum stress a bolt can withstand without breaking
Yield strength = the stress at which a bolt starts to deform permanently

Grade 2 Bolts

• Use Case: Light-duty applications—furniture, household repairs, non-critical automotive work

• Pros: Low cost, easy to install

• Cons: Not suitable for heavy loads or structural components

Grade 5 Bolts

• Use Case: Automotive assemblies, machinery, medium-load structures

• Pros: Stronger than Grade 2, widely available

• Cons: May corrode in harsh environments without coating

Grade 8 Bolts

• Use Case: Heavy machinery, suspension systems, structural joints

• Pros: High strength, reliable under extreme stress

• Cons: More expensive, brittle under some conditions

How to Identify Bolt Grades by Markings

Most SAE bolts have radial lines on the head indicating the grade:

• Grade 2: No markings or “2”

• Grade 5: Three radial lines

• Grade 8: Six radial lines

Tip: These markings are raised, not stamped, and are visible even on coated bolts.

Metric vs. SAE Bolt Grades
Metric bolts use property classes instead of grades (e.g., 8.8, 10.9, 12.9). A metric 10.9 bolt is roughly equivalent to a Grade 8 SAE bolt in terms of strength.

Choosing the Right Grade for Your Application

Ask the following questions:

• What load will the bolt bear? (Static vs. dynamic load)

• Will it face vibration or impact? (Use lock washers or threadlockers if so)

• What’s the environment? (Wet, salty, hot = corrosion-resistant coatings or stainless options)

• Is failure a safety hazard? (Always over-specify in safety-critical applications)

Tips for Using High-Strength Bolts

• Use a calibrated torque wrench to avoid over-tightening

• Don’t mix bolt grades in the same assembly

• Always match nuts and washers to bolt grade (e.g., Grade 8 bolt → Grade 8 nut)

• Consider preloading and bolt stretch in high-load applications

Conclusion
Choosing the correct bolt grade is critical to ensuring safety, structural integrity, and long-term performance. Whether you’re bolting down a motor, assembling a machine, or securing a steel structure, understanding the differences between Grade 2, 5, and 8 bolts helps you make the right call.

At Ironridge Supply Co., we carry a full inventory of industrial fasteners—including SAE Grade 2 to Grade 8 bolts, metric alternatives, washers, and threadlockers. If you're unsure which grade fits your application, our team can help spec the right product for your needs.

🧰 Need Help Identifying a Bolt? Contact Our Fastener Experts

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