M8 button head: Zinc finish Flat Socket head cap screw M8x 1.25 pitch, uses M5 allen wrench to tighten/loosen. The nut size is M8, which uses a 13mm wrench/socket to adjust. These bolts typically used on bodywork, etc.
*To determine the length of a buttonhead cap screw, please measure the bolt shank from under the head in millimetres (do not include the head)