[SleepyC]

That distance is just a suggestion. Your going to want to space the motor accordingly so that you make the proper CG with that motor.

The black fiberglass is there to help baffle the motor. If your running a little gasser, you would just want to cut the bottom out right in front of the cooling fins. And a larger hole behind the motor so that the rear hole creates a low pressure area and pulls cool air through the fins and out the hole, dropping the temp of your motor.