Designers and contractors use many different fasteners with different head markings for many different applications. The markings indicate a fastener’s grade and strength (material properties), aid in easy identification and accountability, and even tell its origin. Society of Automotive Engineers (SAE) fasteners have radial lines for identification. American Society for Testing and Materials (ASTM) uses a combination of letters and numbers to indicate the grade. The markings can be either raised or depressed.
Know the different grade markings and property class markings to select the correct fastener
Examples of SAE J429 fastener grade markings. (Photo courtesy of Industrial Fasteners Institute)
As the grade of the fastener increases, so does its strength. Three radial lines spaced at 120 degrees indicate an SAE grade 5 fastener made of medium-strength carbon steel. Six radial lines spaced at 60 degrees indicate an SAE grade 8 fastener made of hardened, medium carbon-alloy steel. Grade 5 and grade 8 fasteners are heat-treated, quenched and tempered to attain the desired strength. These are the most common grades, but there are others.
Example of an ISO-3506-1 stainless steel fastener head marking. (Photo courtesy of Fastenal Co.)
Grade identification markings on metric fasteners per ISO 898-1. (Photo courtesy of Fastenal Co.)
Grade 2 is standard hardware grade steel. Lower-grade fasteners will stretch more easily than higher-grade fasteners. Some fastener heads don’t have lines. If there are no numbers or lines present on the fastener, assume it is no better than a grade 2 (low strength). These will not pass construction standards, but may be fine for household repair. Smaller fasteners—less than a quarter inch—are generally too small for identification marks.
Often a fastener head will have a two- or three-letter code, which is the manufacturer code (for example, HKT stands for Hau Kwang Enterprises). These codes mean the fastener is registered and accountable, and can be referenced in an identification guide book. A fastener uniquely marked by its maker can be traced back to its manufacturer should a problem occur. The lettering can be moved around on the head to accommodate the grade markings.
There are also differing marks for metric fasteners and those measured in inches. Metric fasteners hold a property class designation (often just “class”), instead of fasteners measured in inches stamped with a grade identification mark. The metric property classes of strength are defined by two numbers separated by a dot, where increasing numbers represent an increasing tensile strength. The number to the left of the dot indicates the ultimate tensile strength in megapascals (mpa) divided by 100. The number to the right is the ratio between yield strength and tensile strength. Fasteners with property class 8.8 have a minimum tensile strength of 800 mpa and yield stress of 0.8×800=640 mpa.
Contributors to this article include Independence, Ohio-based Industrial Fasteners Institute and Winona, Minn.-based Fastenal Co.
