Hex head cap screws, used to join two or more parts together, come in different sizes and grades. Steel hex cap screws range in diameter from 1/4 inch to 1 1/2 inches (0.6 to 3.8 centimeters) and from 3/8 inch to 24 inches (1 to 61 centimeters) in length; however, the very short sizes don't come in wide diameters, and the very long sizes don't come in thin sizes. The Society of Automotive Engineers (SAE) has set the standards for different grades of hex head cap screws according to their strength. The grades are two, five and eight. Although grade nine is also sold, and is assumed to be around 20 percent stronger than grade eight, it isn't an SAE grade and doesn't have standard chemical and physical properties the way the other grades do.
A grade five hex cap screw has more strength than a grade two, but less than a grade eight. A grade two hex cap screw is made from medium or low-carbon steel, a grade five consists of medium carbon steel, and a grade eight is made out of medium carbon alloy steel. How can you tell just by looking at a screw what grade it is? In addition to the head of the screw carrying the manufacturer's symbol, a grade tow has no other markings, a grade five has three equidistant lines radiating out from the center, and a grade eight has six equidistant lines radiating out.
In terms of tensile strength, a grade two hex head cap screw can handle between 60,000 to 74,000 pounds of pressure per square inch (psi), a grade five handles 105,000 to 120,000 psi, and a grade eight should be able to manage 150,000 psi. The core hardness of grade five hex head cap screws, measured in Rockwell, can range from C19 to C30, while the core hardness of grade-8 hex cap screws should be from C33 to C39.