Originally Posted by
User1
OK so if a 4mm wrench does not turn a M4 bolt and a 5mm wrench does not turn a M5 bolt. Then what does a 4mm wrench and a 5mm wrench turn?
(I'm assuming I'd be able to deduct what a 6mm wrench turns after the answer.)
Somehow I don't think you'll be able to deduct anything- for basic socket caps, a 4mm key turns an M5, a 5mm key turns an M6 and a 6mm allen key turns an M8 (eight). IIRC for button heads you go down another mm of allen key size.
Other than whitworth fasterners, which nominally sized the wrenches to the diameter of the fasteners, you'll pretty much always have these discrepancies in bolt diameter vs tool size.