There are hundreds of shapes & sizes of nuts. Nuts have internal threads that correspond to the external threads of a bolt. The type of steel and the treatment of the material during manufacture determine the strength & ability to perform. It is important to note that grades & finishes of nuts MUST match those of the accompanying bolt. Hex nuts are made to SAE (Society of Automotive Engineers) standard J429, various ASTM (American Society for Testing and Materials) standards and ISO (International Organization for Standardization) standards. Examples of SAE J429 standards are Grade 2, Grade 5, & Grade 8. Examples of ASTM Standards are A563 and A194 with grades of A, C, DH & 2H. Examples of ISO standards are property classes 8.8 and 10.9. The nut’s width size is determined by measuring across the flats of the hexagon. The nut’s width determines the wrench or socket size to use when tightening. Hex Nuts come in a variety of finishes. Plain finish, zinc plated, yellow zinc plated and hot-dipped galvanized are most common. Use Hot-Dipped Galvanized Nuts with Hot Dipped Galvanized Bolts ONLY because the threads of the nut are tapped oversized to account for the heavy galvanized coating on the bolt. Be sure to match the grade & finish of your nut with the bolt for a proper assembly.