Neu 1512 pinion fitting issue

[wallot]

the motor has so huge diameter that i am able to fit only 18T pinion

i need to use also 16T and 14T but cant position the motor as it already touches the transmition and cant move any further.
for 16T i need to move the pinion closer to the spur about 1mm and for 14T 2mm
any advice?

i am thinking of sanding the transmition case but dont think that I can take 2mm out of it

18T
14T
motor