When you hold a bolt in your hand, its appearance does not always tell you its true strength or intended use. The physical look of a fastener can deceive you, leading to serious consequences if you choose the wrong one for your job. Engineers and manufacturers rely on specific grade markings to understand a bolt's mechanical properties, which directly relate to its tensile strength and material composition. Knowing how to tell what grade a bolt is gives you the ability to select the right fastener for structural integrity and safe performance.
Identify Bolt Grade Markings on the Head
You can find the most common way to identify a bolt’s grade by examining the markings stamped on its head. These markings serve as codes that reveal the fastener's strength and material properties. The American Society of Automotive Engineers (SAE) uses a system of radial lines to show the bolt's grade. A Grade 2 bolt has no markings, a Grade 5 bolt shows three radial lines, and a Grade 8 bolt has six lines. These clear visual indicators help you quickly determine a bolt’s strength at a glance, helping you choose the right one for the job.
Understand Common Metric Bolt Property Class
The metric system uses a different method for grading bolts, which involves a two-number code stamped on the head. For example, a bolt with the number 8.8 represents a medium-strength fastener. The first number, 8, indicates the tensile strength in megapascals, while the second number, .8, indicates the yield strength as a percentage of the tensile strength. The higher the numbers in the code, the stronger the bolt. Common metric grades include 8.8, 10.9, and 12.9, which help you choose the correct bolt for your application.
Decode Imperial Bolt Grades With Lines
Imperial bolts, standardized by the Society of Automotive Engineers, rely on a pattern of raised radial lines to show their grade. A Grade 2 bolt has no markings on its head and serves for general purposes with low strength requirements. A Grade 5 bolt features three evenly spaced radial lines and provides a stronger option for most automotive work. A Grade 8 bolt has six radial lines, making it the strongest fastener for heavy-duty and high-stress applications that require maximum performance.
Recognize Stainless Steel Bolt Identification
Stainless steel bolts often use different marking systems, which include letters and numbers to identify the alloy. You find common markings like “A2” and “A4” on the heads of stainless steel fasteners. The "A2" designation corresponds to 304 stainless steel, which offers good corrosion resistance for general purposes. The "A4" marking represents 316 stainless steel, a material that provides superior corrosion resistance due to the added molybdenum content. Some fasteners may have a different number following the letter, like “A2-70,” which specifies the tensile strength.
Use Material and Hardness for Unmarked Bolts
Sometimes you find a bolt without any grade markings, which makes identification more difficult. In these cases, you can identify a bolt's grade by knowing its material and hardness. An unmarked bolt usually falls into the Grade 2 category, which indicates a low-strength fastener suitable for non-critical uses. You can also use a material hardness test to determine its properties. You can test the bolt with a file to see if it bites into the metal, which helps you understand its relative hardness and general strength level.
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