How many lines are on the head of an SAE grade 7 bolt?

The head of an SAE grade 7 bolt is marked with five radial lines. These lines serve as a visual indicator of the strength classification of the bolt. Specifically, the markings indicate that the bolt has a minimum yield strength of 230,000 PSI, which places it in the medium-strength category. Understanding these markings is crucial for engineers and technicians when selecting the correct bolt for applications that require specific strength characteristics.

The options for different numbers of lines correspond to various grade classifications, each having its own strength properties and markings. For instance, an SAE grade 5 bolt would typically have three lines, whereas grade 8 would have six lines. This systematic approach to bolt marking helps ensure that the appropriate fastener is used in the proper application, thus maintaining safety and performance standards in mechanical assemblies.