I'd like to replace the original bolt by a security bolt to make it more difficult for thiefs to steal my saddle. The thread of the original bolt has a diameter of 3mm and can be tighten and untighten with a 4mm hex key. The bolt is 20mm long (this includes the 4mm head).
What is the thread pitch? Judging by common sizes it would be 0.5mm or 0.7mm.
Assuming the thread is 0.5mm, would the right bolt be M3 x 16mm (and M4 x 16mm if the thread is 0.7mm)?
Thank you for your help.