The bolt size for the mirror assembly on a 2014 Kawasaki Ninja may vary depending on the specific model and version of the bike. However, based on the information provided, it's likely that the bolt size is M6 (metric) rather than 5/16" (imperial) or 8mm.
M6 bolts have a thread diameter of 6mm and are commonly used for motorcycle mirror assemblies. If you are experiencing difficulty fitting a 5/16" or 8mm bolt into the base threads, it's possible that the threads may be damaged or there could be a different thread size or pitch. In such cases, it's advisable to consult the owner's manual of your specific Ninja model or contact a Kawasaki dealership or authorized service center for accurate information and assistance. They can provide you with the correct bolt size and help resolve any issues you're encountering.