There are six things that can be told by looking at the head of a bolt :the type of fastener or drive type ,the grade of the fastener, the manufacture, the material ,the plating or coating and if it is a fractional or metric fastener. These are broken down into groups called grades that classify them according to rank, quality ,degree, and the other facets. According to SAE specifications ,as the grade of the fasteners increases, so does the bolt’s strength. The grade of a bolt and its manufacturer are identified on the head .Lines of varying number and position on the head show grade .The basic rule of thumb for determining grade is to count the number of lines on the head and add 2; that’s the grade.fig show the detail.
Grade 2 bolts are the soften grade of steel bolts. There are no marking on the head(0 + 2=Grade 2).
A Grade 5 bolts can be identified by the three lines. Grade 5 is head treated. It is made of medium carbon steel, quenched and tempered.
Grade 8 can be identified by the six lines. It consists of medium carbon alloy steel, heat-treated, quenched and tempered. It is a hardened bolt.
Non-radical head Markings:
Some head marking don’t follow the “Number of lines”. It is important to know the different grade markings or property class marking to prevent error in identifying bolts as the kind of steel that bolts are made from and treatment they receive during manufacture determine their strength and their strength and their ability to do their job.
A 307 A bolt can be identified by the self –identifying stamp. 307A bolts are a Grade 1 equivalent that is used in low-strength application. It has lower hardness and tensile requirement than grade 2 bolts. It is generally used in low-pressure, standard or normal temperature applications and is made of low carbon steel.
A Grade 9 has no industry standard specification for its head marking, and therefore the design is up to the manufacturer .However, the head usually have nine dots or lines .These bolts are designed for use in high-stress and high-strength applications.