Metric fasteners come in all manner of shapes and sizes. In order to efficiently organize the many different metric fasteners that are available, suppliers and professionals who use them follow a standard nomenclature. Here's a guide to understanding the metric fastener nomenclature.
The Standard Name
The standard name for a metric fastener follows a basic formula. The formula can be thought of as M(a)-(b)(c), where each of the lowercase letters is a different numerical variable. Letters "a" and "c" are typically expressed as whole integers, and letter "b" is usually stated to the tenths.
For example, a metric fastener might be named M6-0.8x15. This name will be used below to understand what each of the different parts means.
The Letter M
The names of all metric fasteners start with a capital letter "M" in order to differentiate these fasteners from fasteners that use the U.S. system. Fasteners that use the U.S. system are commonly called inch fasteners.
If you're using fasteners for the first time, either inch or metric fasteners will work for your project. If you're replacing fasteners, you'll need the same type of fastener as was originally used. Metric fasteners won't be easily compatible with parts that originally had inch fasteners, and the opposite holds true as well.
The First Number
The first number immediately follows the M, and it details the fastener's diameter in millimeters. The higher the number is, the thicker the fastener will be — and you'll need a fastener that has the same diameter as the hole that you're securing it through.
In the given example, the diameter of the fastener is 6 millimeters. Importantly, this is the diameter of the fastener's shaft and not its head.
The hyphen is merely a placeholder that makes the name of a metal fastener easier to read. The mark serves no purpose other than increasing readability. It does make long lists of fastener sizes much easier to peruse.
The Second Number
The second number follows the hyphen. This number expresses the pitch of the fastener, which is the distance between the fastener's threads. The number is given in millimeters to the nearest tenth-millimeter.
The pitch of the example fastener is 0.8 millimeters. A fastener's pitch must match the previous fastener's pitch if you're replacing a fastener.
The Third Number
The third number of the fastener's name is the length of the shaft, again expressed in millimeters. The shaft of the example fastener is 15 millimeters long.