for ideal meshing;
- take a bit of paper (thin like newspaper)
- put it between spur and pinion and mount the motor in such a way that the paper sits tight between the pinion and the spur, but still will not be torn apart when you turn the spur of pinion but comes out wrinkled.
- remove the paperstrip.
your pinion and spur will now be tight but with a very tiny amount of play and that's what you are looking for.