ever wondered what size nut an M12 was? or what nut to buy for a bolt that takes a 17mm socket? heres a quick list of the main ones: M4 7mm M5 8mm M6 10mm M8 13mm M10 17mm M12 19mm M16 24mm prob useless info but might come in handy.
Here are the imperial equivalents too http://www.classicmotorcycles.org.uk/technical/technical_spanner_size_chart.htm
In general that's handy pablo most jap cars will use a 12mm head instead of a normal 13mm also some will have metric fine threads.