How to look at screw models
That is, there are certain specifications of the national standard, such as: M6, M8, M10, M12, M16, M20. Which 7911131415171819 there is no. Wrench it as such as: M6 wrench, M16 wrench and so on.
According to the size of the outer diameter, such as the outer diameter of 6 mm, then the M6 screws. But not every millimeter has, according to a certain law.
When you buy a certificate. The The Written above. The The I do not know any other way
