You are standing in a hardware store aisle. Or maybe you are staring at a pile of loose hardware in your garage. You need to join two pieces of steel together. You grab a hex head cap screw. But which one?
The difference between a cheap screw and a structural one isn’t just price. It is strength. It is chemistry. It is the lines stamped into the head.
Hex head cap screws come in a bewildering array of sizes. Steel versions range from 1/4 inch to 1 1/2 inches in diameter. Lengths stretch from 3/8 inch all the way to 24 inches. But there is a physical limit. The very short screws don’t come in wide diameters. The very long screws don’t come in thin sizes. Physics gets in the way.
The Society of Automotive Engineers (SAE) sets the standards. They define the grades. Specifically, they define Grade 2, Grade 5, and Grade 8.
There is a Grade 9 screw out there. You might see it. It is assumed to be about 20 percent stronger than Grade 8. But it is not an SAE grade. It lacks standard chemical and physical properties. Stick to the official three unless you have a specific reason not to.
Reading the Head Markings
How do you tell them apart without a lab test? You look at the head.
Every screw carries the manufacturer’s symbol. That is the baseline. But the lines tell the story of strength.
Grade 2 is the entry level. It has no other markings. Just the logo. If there are no lines radiating from the center, it is Grade 2.
Grade 5 has three equidistant lines. They radiate out from the center like a Mercedes logo. Or a fork. Three lines mean medium carbon steel. It is stronger than Grade 2. It is weaker than Grade 8.
Grade 8 has six equidistant lines. More lines. More strength. These are made from medium carbon alloy steel. They are the workhorses of heavy machinery and automotive applications.
The visual cue is simple: no lines means Grade 2. Three lines means Grade 5. Six lines means Grade 8.
The Numbers Behind the Steel
Strength is measured in tensile strength. This is the pressure per square inch the screw can handle before snapping.
Grade 2 handles between 60,000 and 74,000 psi. That is sufficient for light duty. Furniture. Interior trim. Things that don’t shake or vibrate much.
Grade 5 jumps to 105,000 to 120,000 psi. This is where things get serious. Automotive frames. Suspension components. Structural bolts.
Grade 8 manages 150,000 psi. This is high tension. Heavy equipment. Industrial machinery. If you are building something that needs to hold together under extreme stress, this is the bolt.
Hardness matters too. We measure this in Rockwell C.
Grade 5 core hardness ranges from C19 to C30. Grade 8 ranges from C3
