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Any body can tell what is the advantage of different of cut ? (as this photo is too small & I can't read them) Thx !

The only advantage I know of is to advance the timing on fixed timing motors,like the bottom diagram,not sure on the others.

of the 4 cuts pictured the top 3 produce less drag on the motor. the 4th one is a timed brush which will allow you to slightly change the timing of the motor to produce more power.

downsides #1 and 3 from the top is uneven com wear. #2 and 4 the come wears even. but in all 4 cases the brushes wear down and need replaced a lot sooner. back in the day it was not common to do in club races among the poorer racers due to costs. i had tried all combinations at one time but due to my driving skill (or lack of) it didn't offer me any advantage.

Pic 2 and slightly fileing the edges rounder way less drag and motor kicks up or revs up faster.Playing with different springs for brushes helps but really need a motor dyno to tell if any of the steps helps.I got all my motors coms turned and dynoed then stopped racing now just sitting in a storage ben.Don

i have always had great results with a #42 hole in the center of brushes.