Fastener manufacturers grade and mark bolts according to the standards of several organisations, including the American Society for Standards and Testing (ASTM), Society of Automotive Engineers (SAE) and the International Organization for Standardization (ISO). Bolt grade markings appear on the top of the bolt's head and typically consist of lines, shapes, letters or numbers. Bolt grades indicate important characteristics, such as strength and hardness, and allow construction professionals and mechanics to choose fasteners that won't fail under stress.
Grade 8 Marks
Manufacturers stamp six lines atop the heads of grade 8 bolts. Each of the six straight lines begins near the centre of the bolt's hexagonal head. The line continues outward until it meets with the apex between two of the head's sides. Grade 8 bolts are one of the strongest types of bolts. To qualify as grade 8, a bolt must consist of alloy steel or carbon alloy steel. Grade 8 metals undergo metalworking processes called quenching and tempering.
Grade 7 Marks
Grade 7 bolts have five lines stamped upon their heads. The five straight lines are equidistant but don't always align with the apex between the bolt head's sides. Grade 7 bolts are slightly less strong than grade eight bolts. Grade 7 bolts are made of carbon alloy steel, quenched and tempered.
Grade 5 Marks
The stamp upon the top of standard, grade 5 bolts features three equidistant lines. The lines begin near the bolt head's centre and continue outward to intersect with the apex between two of the head's sides. Grade 5 bolts are the weakest of the common bolt grades. Grade 5 bolts are made of medium carbon steel, quenched and tempered.
5.1 and 5.2 Marks
These 5.1 and 5.2 sub-grades apply to speciality fasteners. Grade 5.1 bolts have a set of three lines stamped upon their head. The T-shaped arrangement of the grade 5.1 bolt's marks distinguishes it from the standard grade 5 bolt. Grade 5.1 typically applies to pre-assembled fasteners, such as an integral bolt and washer combination. Like grade 5.1 bolts, grade 5.2 bolts feature a set of three lines. However, the 5.2 bolt's lines remain on one side of the bolt's head to form a rough W shape. Grade 5.2 bolts consist of low carbon, martensitic steel. The special steel used to create grade 5.2 bolts increases the fastener's hardness.