The underhead knurling has two distinct functions. Once the screw has been tightened, the indentation causes the unscrewing torque used to be greater, making the fixing more stable in all those applications which are subjected to movements and / or vibrations during their use. In the electric field, however, the indentation scrapes the surface of the metal, effectively creating connectivity with the material where grounding is required. However, it must be taken into account that the serration can never replace the quality of the tightening of a screw with the serrated washer, in itself much more performing.
The length of the screw is measured from the tip to the underhead.