Hey Metric Heads, we Americans are not as familiar with your system as we probably ought to be. Or at least this American isn't.... :D Here's my situation: I'm playing with the FunnyBone drawings and looking at common metric wrench sizes. Now, the ideal size progression on my tool is 6,8,10,12,14,16,18mm. It is my understanding though that 13mm is a very important and common size. It also happens to be very close to our American 1/2" which is an extremely common size here so I can see the attraction. My question is, how critical is 13mm? The reason...