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Grade Markings Help You Choose the Right Fastener for Your Project
adapted from InCom
Distributor Supply
Head markings on cap screws are your assurance that the fastener you choose will do the job it's designed to do.
The Society of Automotive Engineers, American Society for Testing and Materials, and the Industrial Fasteners Institute have established specific ranges of hardness and tensile strength for each grade of fastener.
There are certain applications which engineers have determined are appropriate for the various grade of fasteners. Do-it-yourselfers can use the grade markings to be sure the bolt or cap screw will meet their requirements.
Common Cap Screw Grade Markings Grade 2
(No marks)
Use for wood and general hardware applicationsGrade 5
(Three radial lines)
Use for small engine repair and for automotive needsGrade 8
(Six radial lines)
Automotive needs and other applications where high tensile strength is required
Grade 2 Fasteners
Often referred to as "hardware" quality, these fasteners are typically made of low-carbon steel. They are ideally suited for holding wood pieces together (in combination with appropriate nuts and washers) or general hardware use where higher strength is not required.
There is no grade marking on the head of Grade 2 fasteners, although many manufacturers will put a distinguishing company identification on the head.
Grade 5 Fasteners
Grade 5 fasteners are made of medium-carbon steel. These fasteners are then quenched and tempered for the additional strength necessary for most automotive uses and other applications where strength is a moderate concern.
The grade marking on the head of a Grade 5 fastener is three equally-spaced lines coming out from the center of the head. Manufacturers' identifications are added for traceability.
Grade 8 Fasteners
Grade 8 fasteners are manufactured of medium-carbon alloy steel for the most demanding applications. These fasteners are then quenched and tempered to superior strength and hardness qualities. The grade marking on a Grade 8 fastener is six equally spaced lines coming out from the center of the head. Once again, the manufacturer's mark is necessary for traceability.
Do not confuse the use of Grade 8.2 boron bolts with true Grade 8 fasteners. The two grades have different characteristics and should not be used interchangeably.