Type of thread on 2 speed pinions

I have trouble because the brake disc will hit the motor if I go any further forward. I'm using a servo for the brakes to try and increase run time and to keep the temperatures down (I'm going to be racing against other gas powered 1/8 cars flat out on a purpose built track for 15mins). I agree with you on having the pinions that far out but I will just have to see how it goes.