Info:- Metric Bolts Thread Size vs Head Size
There's often some confusion over the the size of bolts/nuts and spanner sizes.
Metric bolts are designated by the thread size, so a European 12mm bolt or nut will need a 19mm spanner.
Unfortunately there are 3 systems in use, ANSI/ISO (American) DIN (European) and JIS (Japanese)
Hopefully the cross reference chart below will help to clear things up
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