Grades for nuts and bolts

▶ It’s important to know what type of nut or bolt you are fitting or has been fitted, especially to a safety-critical component such as a brake caliper. The head of a bolt may have a number or letter (or both) displayed on the top. The following information helps to explain what this means.

A2: Stainless steel nuts and bolts made from steel with around 18% chromium and 8% nickel to offer a high level of corrosion resistance. Not recommended for safety-critical components.

A4: A better standard than A2 (contains the same chromium and nickel, but also 3% molybdenum). Better corrosion resistance so generally more expensive than A2, but longer lasting. However, this is a stainless steel fastening and should not be used on safety-critical components.

4.6/8.8/10.9/12.9: A metric grading displayed on the top of the bolt’s head, which indicates its tensile strength. The higher the number, the greater the bolt can be tightened, making it more useful for applications where a high torque setting is required. 8.8 is now standard for most applications including safety-critical components, although 10.9 is commonly used in critical parts of a vehicle and 12.9 is found on high-performance vehicles.

SAE 2/5/8: Found on Unified fastenings and similar to the metric grading mentioned above for 4.6, 8.8 and 10.9. This is very confusing because the SAE grades are denoted by radial lines on the head of the bolt. And the number of lines doesn’t represent the SAE grade. No lines means SAE 2, three lines represents SAE 5 and six lines is for SAE 8. And there’s more. The nuts to go with these grades have a series of letters from A to C, which match the SAE grades from 2 to 8.