Hex Cap Screw Buyer's Guide: Know Your Grade
When it comes to replacing a hex head cap screw, do you know how to determine the grade of the cap screw and what that “grade” actually means? This guide is designed to show you how to quickly and easily identify the grade of any hex cap screw and understand the meaning and uses for each grade and material.
How to Determine the Hex Bolt Grade
Hex bolt grade refers to the grade of steel used in manufacturing the screw. This determines the amount of stress that can be applied to the fastener, known as "tensile strength."
The grade of a hex cap screw can be identified by markings on the head of the screw. But first, why are there standard markings on the head to indicate grade? Well, Congress passed a law in 1990 that set out standards for fasteners sold in the United States. It's known as Public Law 101-592, and its purpose was to provide the general public with standards to easily identify a fastener's strength, hardness, grade and composition.
There are three classifications of fasteners in general - SAE, Metric and ASTM
SAE Hex Cap Screw Grades
The image above shows markings (radial lines) on the top of a hex head for grades 2, 5, 8 and L9. These markings are standard. For example, all grade 5 bolts will have three radial lines on the head. All grade 8 bolts will have six radial lines on the head. Watch the video below for more detail.
Grade 2 or Lower
If the cap screw has no markings on the head, it means it is made of low to medium carbon steel. These are the most inexpensive bolts on the market and intended for standard household use.
Hex heads with three markings are grade 5 fasteners. This grade is made from medium quality carbon steel. These are commonly used in automotive applications. Grade 5 cap screws are hardened. During manufacturing, they are heated in order to strengthen the outside of the screw. This makes the outside of the fastener stronger than the inside of the fastener.
With a tensile strength of 150,000 pounds per square inch, grade 8 hex cap screws are stronger than grade 5. This grade is heated similar to grade 5, but they are fully heat treated versus case hardened. Their strength makes them good candidates for tractor fabrication, suspension applications and other high demand fastening applications.
Grade L9 fasteners are made from high quality alloy steel. With a tensile strength of 180,000 pounds per square inch, L9 grade hex cap screws are perfect for high stress environments.
Metric Hex Cap Screws Grades
The image above shows hex head markings for 8.8, 10.9, 12.9, and 18.8. These markings are standard on metric size fasteners and refer to the tensile strength.
Metric Grade 8.8 Hex Cap Screws
Grade 8.8 fasteners are made from heat treated, medium quality carbon steel. They perform well in most applications, and are most commonly used in motors, engines and industrial equipment.
Metric Grade 10.9 Hex Cap Screws
Grade 10.9 fasteners are similar to grade 8. They can be made from medium quality carbon steel, a carbon steel alloy, or from a low-quality boron steel. With a high tensile strength these cap screws are often used in automotive applications.
Metric Grade 12.9 Hex Cap Screws
Used in high stress environments, grade 12.9 fasteners are the highest class of metric cap screws. Where Grade 8 fasteners have a 150,000 psi tensile strength, 12.9 fasteners have a 180,000 psi tensile strength, making them more durable than the grade 5.
ASTM Grade Bolts
ASTM refers to the grade standards set by American Society for Testing and Materials. Typically, the strength of the fastener is notated by the letter "A" plus three digits, such as A307.
A2/Type 304 (18-8) Stainless Hex Cap Grade
Type A304 fasteners are the most commonly used stainless steel fasteners on the market. Consisting of a standard of 18% chromium and 8% nickel, it's perfect for high temperature applications and is resistant to corrosive acids. They can be used in freshwater marine environments, but are not well suited for saltwater environments.
A4/Type 316 Stainless Hex Cap Grade
A4/316 stainless fasteners are also known as marine grade fasteners due to their ability to withstand the corrosive nature of salt water. These cap screws are good for applications such as docks, piers, boats and boat houses.
A-307B Grade Cap Screw
These cap screws are known as "heavy hex cap screws." They have a larger head compared to standard hex heads and are shorter in length. A-307B cap screws can be used in many applications, including wood and steel. They are used for fastening structures such as bridges and piers, and also used for pipe flanges.
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