A Vernier caliper is a graduated sliding clamp usually ... A micrometer is a G clamp with jaws in its most common form around 25 mm in size, and with a very accurately calibrated screw thread.

I'm not sure what size the bolt is, so I'll need to measure it. I've got the vernier caliper as it allows me to take really accurate measurements. I'll then need to get the new bolt and replace it.