Which special marking on the head of a bolt indicates the quality of the fastener?

The marking on the head of a bolt that indicates the quality of the fastener is related to its grade. Grades are typically represented by numerical markings or other symbols stamped on the bolt head, which signify the bolt's material properties, tensile strength, and intended use. Higher-grade bolts are designed to handle heavier loads and provide greater strength, making them suitable for critical applications in construction and manufacturing, including boilermaking.

For example, a bolt marked with a grade such as 5 or 8 means it meets specific standards set by organizations like ASTM (American Society for Testing and Materials) or SAE (Society of Automotive Engineers) for hardness and tensile strength. Understanding these markings helps professionals in the field select the appropriate fasteners for their projects, ensuring safety and performance.

The other choices do not serve this purpose. "Machine" refers more to the type of fasteners used in specific applications rather than indicating quality. "Cap" is associated with cap screws, which are designed differently and don't specifically convey quality or grade. "Stud" refers to a type of fastener with threads on both ends and does not indicate quality markings. Thus, the correct choice highlights the importance of recognizing and understanding bolt grades for effective and safe usage in construction and assembly