Bolts are made of different grades of steel. The greater the steel's tensile strength, the more torque the bolt can take and the tighter the bolted joint will be. Standards for bolt strength grades are set according to a system devised by the Society of Automotive Engineers; this marking system uses raised dashes on the bolt head to indicate strength. Other systems use raised numerals on the bolt head to indicate bolt grade.
SAE American Grades
The SAE marking standard starts with grade 2, indicated by a bolt head with no markings whatsoever. A grade 2 bolt made of low-carbon steel has a tensile strength of 64,000 pounds per square inch or less. Tensile strength is the amount of pull the bolt can withstand before breaking. A bolt head with three raised dashes in a radial pattern marks an SAE grade 5 bolt of tempered medium carbon steel with a tensile strength of at least 105,000 pounds per square inch. The strongest commercial-quality bolt is grade 8, marked by six raised dashes; its medium-carbon alloy steel has been quenched and tempered to achieve a tensile strength of 150,000 psi.
The strength grade of a metric bolt, known as its property class, consists of two numbers separated by a dot. The property class is expressed in raised or depressed numerals on top or on the side of the bolt head, according to rules set by the International Standards Organization. The first number represents the load in megapascals -- a pascal unit of measurement for internal stress -- required to break the bolt. The second number represents a ratio between breaking load and bending load.
Higher Number Means Stronger
The higher the ISO numbers are, the stronger the bolt. A metric bolt designated as ISO class 6.8 roughly corresponds in strength to an SAE Grade 2 bolt. Bolts of ISO class 8.8 and the slightly stronger class 9.8 roughly correspond to an SAE Grade 5 bolt. An ISO Class 10.9 bolt roughly corresponds to an SAE Grade 8 bolt.
Another widely-used system of bolt strength grades comes from ASTM International, formerly the American Society for Testing and Materials. Its strength grades are indicated by the letter A plus three numerals stamped on the bolt head. Common ASTM grades include A307, which roughly corresponds to SAE Grade 2. An ASTM A325 bolt is roughly equivalent in strength to SAE Grade 5 and an A490 bolt is about equivalent in strength to SAE Grade 8.