I have to admit, its hard to figure out what size bolts or thread pitch when working on your car.  To make my life just a littl easier, I always keep this metric thread pitch gauge in my tool box. This tool is used to measure a bolts diameter, pitch, and length. Pitch ranges include .8 to 6.0 . Length measurements from 5 to 230mm, and diameters from 4mm to 20mm.  A great tool to have.