After several years in the trade and not a few weeks over that period spent struggling with recalcitrant vehicles, I found out something I should have known years ago.
I went to a builder's suppliers to get a couple of 12 mm bolts. So the man comes back with a couple that are about 1/4 inch too large for my needs.
"No; I want something about 2/3rds that thickness." I say, astonished.
"That'll be 8 mm then." "Whuh.. It will?"
I think about this as he comes back with two half inch bolts that are exactly the right size.
I knew that. Smarra-me? I didn't half feel silly!