As a subject matter expert in mechanical engineering, I can tell you that when you see "m8" on a bolt, it typically refers to the nominal diameter of the bolt. The "m" stands for millimeter, which is the unit of measurement for the diameter. So, an "m8" bolt has a diameter of 8 millimeters. This is a standard designation used in metric measurements for fasteners.
However, it's important to note that the actual major diameter of an M8 bolt, according to ISO standards, is slightly larger than 8mm due to tolerances. The minor diameter and the thread pitch also have specific measurements that are defined by the standard for an M8 bolt.
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