M5 button head: button head Socket head cap screw M5x 0.8 pitch, uses M4 allen wrench to tighten/loosen. The nut size is M5, which uses a 5mm wrench/socket to adjust. These bolts typically used on steering wheels, bumpers, 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)