On my RC18B I have installed a new motor and its a Orion Baja modified motor and I can't get the gear meshed and when i put it in reverse the gear makes a griding noise and no matter where i put the motor or the gear it still makes the griding noise so anybody please help with the gear meshing thanks :)

What pinion did you put on the motor when you installed it?

A 15t or a 13t

Check BRP diff shim kits.

Try moving the diff. shims and tit en up the diff. gears.