All the wholes on the middle of the chassis are intended for motor mount positioning. There are 2 positions for fitting the motor infront and 1 for back. The reason ur gear hits the chassis is because you have the motor mount aligned on front motor mount screws when it’s reversed and you want the motor at the back
Move the spacers on the spur gear or replace them smdh it ain't that hard to figure out damn use different spacers on the spur damn hobao did the hard part making it real easy to switch things around!!!!!!!!