That's a fairly peculiar looking thread. It's almost as if it was cut twice in error. The correct measurement is not 13 mm, though, as you should measure from the underside of the head to the end of the bolt. Also needed is diameter.

Consider to locate a similar diameter bolt from a local source, regardless of head and length, to test fit into the threads. I suspect that this remaining bolt is to be considered defective/faulty. Finding an undamaged bolt that threads in easily will give you precise diameter and thread pitch and simple measurement will give you length, then search for cap head machine screw to match those specifications.