Bolts are composed of different grades of steel, the greater the tensile strength of the steel, the more torque the bolt can withstand, and the more secure the bolted joint is going to be. The standards for the strength grade of bolts are set according to a system ideated by the Society of Automotive Engineers for bolt markings. By utilizing a series of raised dashes on the head of the bolt which indicate the strength of the bolt it makes them readily identifiable to an Inspector or mechanic.
The lowest SAE grade is grade 2 which is made of low-carbon steel and has a tensile strength of 64,000 psi or less. This is the amount of torque the bolt can withstand before it breaks from the stress. This type of bolt has no markings whatsoever on the head.
The next highest grade is the SAE grade 5 bolt which is made out of tempered carbon steel that has a tensile strength of no less than 105,000 pounds per square inch. These bolts have three radial lines on the head going from the center to the sides.
Grade 8 bolts
These bolts are the strongest and most durable bolt fasteners ever made. They are used in high-stress applications, such as valves, pumps, motors, automotive suspension systems, aerospace, military, marine, and heavy construction projects.
Grade 8 bolts have a high tensile strength rating due to its medium-carbon alloy steel composition that has been quenched and tempered in order to achieve a high tensile strength of 150,000 psi. They are no less than 25% stronger than any other bolt when compared to medium-strength steel bolts.
The pre-load strength of grade 8 bolts is greater than that compared to regular bolts and Grade 5 bolts. Here are two ways to identify Grade 8 bolts from other bolts of lower grade.
Use a Magnet
You can easily identify grade 8 bolts from other bolt grades by using a magnet. Unlike other bolts of different grades, grade 8 bolts are not made of stainless steel, which means that a grade 8 bolt will not be attracted to a magnet. Position the magnet in the pile of bolts that you want to classify. The bolts that are not attracted to the magnet are Grade 8 bolts.
Look for Bolt Markings
If you do not have a magnet you can still identify Grade 8 bolts by brushing the head of the bolt using a wire brush and counting its radial lines. There are six radial lines on the heads of Grade 8 bolts. Length is measured from under the head. They're available with steel hex heads as well.
You can check out this Bolt Depot breakdown for more info and sizes.