A machine drawing specifies an M12 screw for assembly. What does 'M12' indicate about the screw?
The major diameter of the screw is 12 mm
In the ISO metric thread system a screw is designated by the letter M followed by a number, for example M12. This is a standardised way of stating the thread size.
The letter M shows the thread is a metric thread, and the number that follows gives its nominal size in millimetres.
That nominal size is defined as the major (outside) diameter of the external thread, the diameter measured across the crests of the thread.
So M12 means a metric thread whose major diameter is 12 mm.
The number does not state the screw length (option 2), which is quoted separately after the size, nor the head diameter (option 3), which is a different feature of the fastener.
It is also not the pitch (option 1); the pitch is a much smaller value given after a multiplication sign, as in M12 x 1.75, and is not the 12 itself.
The single number in the M designation always refers to the major diameter.
Hence, the answer is The major diameter of the screw is 12 mm.
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