Preventive maintenance
Fastener Grades and Torque: SAE and Metric Reference
Inch fastener strength is shown by radial lines on the head — none for Grade 2, three for Grade 5, six for Grade 8. Metric strength is a printed property class such as 8.8 or 10.9, where the number is the strength written out: 8.8 means roughly 800 MPa tensile and 640 MPa yield. A replacement must be equal or greater strength and the correct size, and the nut grade has to match the bolt.