20-10-14, 01:12 PM
They're standard metric screws so they'll be sized the same way as the metric bolts. So you measure the diameter at the threaded end to give you the M size. So 4mm diameter is M4, 5mm is M5, 6mm is M6 etc. The next thing you need is the pitch of the screw. Each screw size has two standard pitches fine and coarse. There is a picture to show the difference. The pitch is basically the distance between the threads so a fine pitch means more threads.
![[Image: thread30.jpg]](http://autobus.cyclingnews.com/tech/fix/torque/thread30.jpg)
The table of standard pitches is there in the "preferred sizes" section. You can see there's two pitch values for each M value
http://en.wikipedia.org/wiki/ISO_metric_screw_thread
So as an example say it measures 4mm at the threaded end so it M4 and by looking at it it looks like a coarse thread so from the table it's pitch would be 0.7 so it M4 x 0.7 and measure the length and it's 20mm and it has a counter sunk head. So it's M4 x 0.7 x 20mm counter sunk screw
The table of standard pitches is there in the "preferred sizes" section. You can see there's two pitch values for each M value
http://en.wikipedia.org/wiki/ISO_metric_screw_thread
So as an example say it measures 4mm at the threaded end so it M4 and by looking at it it looks like a coarse thread so from the table it's pitch would be 0.7 so it M4 x 0.7 and measure the length and it's 20mm and it has a counter sunk head. So it's M4 x 0.7 x 20mm counter sunk screw