A bolt’s grade tells you its strength, and choosing the right one means matching strength to the load without over- or under-building the joint. Two systems dominate.
SAE (inch) grades
Marked by radial lines on the bolt head:
| SAE grade | Head marking | Relative strength |
|---|---|---|
| Grade 2 | No lines | Low (general use) |
| Grade 5 | 3 radial lines | Medium (automotive) |
| Grade 8 | 6 radial lines | High (machinery, suspension) |
Metric grades
Stamped as numbers like 8.8, 10.9, or 12.9 — higher numbers mean higher strength (8.8 ≈ SAE Grade 5, 10.9 ≈ Grade 8). Match the grade to the load and to what the assembly specifies, and use a nut of equal or higher grade so the threads don’t strip before the bolt reaches its rated load. Higher-grade bolts are stronger but more brittle, so they aren’t always right for shock or fatigue.
Frequently asked questions
What do the lines on a bolt head mean? They mark the SAE grade — three lines is Grade 5, six is Grade 8.
Is metric 10.9 strong? Yes — roughly equivalent to SAE Grade 8.
Can I mix bolt and nut grades? Use a nut of equal or higher grade than the bolt.
Why Higher Grade Isn’t Always “Better”
A higher-strength bolt is also a more brittle one. Grade 8 and metric 10.9/12.9 bolts resist stretching and shearing better than lower grades, but they have less give before they crack rather than yield, which makes them a poor choice in joints that see repeated shock or fatigue loading — a slightly lower grade with more ductility can actually outlast a higher grade in that kind of service.
Torque Isn’t One-Size-Fits-All Across Grades
Torque specs scale with grade, diameter, and thread pitch together, not with grade alone — the same 3/8″ bolt carries a very different torque spec at Grade 5 than at Grade 8, and jumping to a stronger bolt without adjusting torque either leaves preload on the table or overstresses a joint designed for a weaker fastener. Reusing an old torque chart after swapping grades is a common source of either loose joints or stripped threads.
Coatings and Plating Change the Torque Too
Zinc plating, black oxide, and dry fasteners all have different friction coefficients, which shifts how much of the applied torque actually becomes clamping force rather than just overcoming friction. A torque spec written for a plain, uncoated bolt can under- or over-tighten a plated one by a meaningful margin, which is why critical assemblies often specify both the grade and the exact coating/lubrication condition the torque value assumes.