When purchasing, repairing or inspecting, how to quickly judge bolt strength? Look at numbers and lines on the head.
Metric head numbers like "8.8": first digit×100 = minimum tensile strength (800MPa); decimal×first digit = yield ratio (0.8), i.e. yield about 640MPa. Common grades: 4.8, 5.8, 8.8, 10.9, 12.9; larger is stronger.
Grade 8.8 is marked "8.8"; 10.9 "10.9"; 12.9 "12.9" often with 3 lines. Unmarked or blurry heads suggest non-standard or low-strength parts—never use in critical joints.
Stainless bolts do not show numbers but "A2-70" or "A4-80": A2/A4 is material, 70/80 is tensile strength (700/800MPa).
The comprehensive fastener platform supplies all grades with material certs.