Bolt Grades Explained

Grade markings on the head of a bolt aren't decoration — they tell you exactly how much load the fastener can carry before it yields or breaks. The most common SAE grades you'll see in industrial work are Grade 2, Grade 5, and Grade 8.

Grade 2 bolts are low-carbon steel with no head markings. They're fine for general-purpose, low-stress connections but should not be used in critical structural applications.

Grade 5 bolts (three radial lines on the head) are medium-carbon steel, quenched and tempered. They're the workhorse of automotive and equipment assembly, with a tensile strength around 120,000 psi.

Grade 8 bolts (six radial lines) are alloy steel and rated to 150,000 psi tensile. Use them where vibration, shock, and fatigue come into play — suspension components, heavy equipment, structural assemblies.

Metric bolts use a class system instead: 8.8, 10.9, and 12.9 are the rough equivalents of Grade 5, Grade 8, and a step above. Never mix metric and SAE on the same joint without checking thread engagement.