[lincpimp]

Use a file, or better yet, a drinding stone. If you can mount the motor, and use the esc to spin it at a slow speed that is the best. If you use a stone, a bit of oil will help to keep the shavings from going everywhere. Try to hold the stone square to the shaft, and use light pressure. A coarse grit stone will work better, as the outrunner should have enough torque.

On a side note most pinions could be drilled out to 4mm. and you could buy a bore reducer from mike, to go from 4mm to 5mm, and then use 5mm bore pinions...