If it is not listed in the rules then it is alowed?
That is why rulebooks are as large as they are now, outrule such things to be legally right.
But yes searching for gaps and limits to use them does make this a good example but it does not belong in the atmosphere of a handout motor or so called "stock racing" It is actually modifying a motor.A break in is indeen a normal thing, sadly due an angle change of the brush it will alter the timing a bit. To prevent people are using many motors due a far break in of the brushes there was with most races I have done a limit of motors to use.