Authored by Stuart Skjerven:
The front end of the arm is known as the "Commutator" or "Com".
The com is made up of 3 plates and these plates form a round circle on which the brushes travel as the arm spins. A true com is one that is perfectly round and all three com plates are the same thickness. If the com is out-of-round it will cause the brushes to bounce on the com as it spins and lose contact, thus causing a power loss.
The higher classes of racing will allow you to use a lathe to true the com.
The only way I have of checking is while the motor is breaking in you look through the little opening in the top of the front bulkhead and gage the amount of blue spark you see where the brushes meet the com. Too much spark indicates a lot of brush bounce which tells you that your com is not very true.
The three most important factors in selecting a good stock, in order of importance, are:
1. a true com
2. a well balanced armature
3. low ohm reading across the com plates
A good racing tip for the G3 and Super G+ is to run the armature with no load for 5 to 10 minutes to break in the brushes. They "Seat" and then help the armature maximize power. This should be done at 6-9 volts on a separate power source.
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