So I have finally gotten around to replacing the negative-engine block ground because the original terminal is getting loose and insufficient. Got the new cable from BA and removed the old one, go to bolt it to the engine block and I lost the bolt. Now this has happened three times and the first two times I got very lucky, only this time I cannot find the bolt. My question is what size bolt is it? I have a hardware store near me that's usually well-stocked on everything even down to the weirdo ones no one ever seems to use.

Just to be clear, it's the bolt used to connect the battery negative to the engine block. I need to know what size it is so I can get a replacement.

Thank you.