On one hand, altering the timing...."during an armature rewind" is very easy to do.
BUT altering the timing on a finished running armature, isn't for the faint of heart. The problem is, the winds of wire are wound tight enough, that as you move the comutator, you are "tightining" the tought wire even tighter.
One of two things will happen -
1. the tight side wires will break.
2. the tight side wires will stretch. When they stretch, they get thin. When they get thin, less power will flow, thus basically negating the comm
. advance operation.
So...what can be done...?
I've found a coupla things that work.
1. Move the magnets in the motor can to "center" the armature in the center of the magnetic field. Remove the armature shims and put the motor back together (WITHOUT the brushes). You can watch the arm move itself to the center. In MOST cases, just shimming isn't enough movement though, you'll need to move the magnets. I've found CA glue good to position the magnets in the can.
2. Shim the armature to limit the arm movement in the assembled can (just like limiting the rear axle side to side movement).
3. This one's a bit tuff - make sure the brushes are "equadistant and square" from the "center" of the comutator.
This can be another problem with doing the work to advance the timing...only to have the brushes offset from center and negate or partially negate all your armature work.
As for "breaking in" brushes, ONLY....do this to the point of the brush having full contact (or near full contact) with the comutator surface.
This is one thing that's been blown all out of proportion (as far as I'm concerned).
UNLESS....you are going to a major race the next day....just do a light breakin (about 6 volts, less than 1 amp) and spend the rest of the "breakin" time with track time....! This is much more important (by me) than 100% brush contact before you put the car on the track..!
One thing that is motor related but not really considered "motor blue printing"...is to shorten the armature so that it does NOT contact the crown gear in inline car setups. Then use tubing to shim the rear axle to center the wheels/tires, and just adjust the crown to get a good crown to pinion mesh.
This removes a fair amount of friction...which is a HP killer...!
This is some of the things I do.
Hope this helps some.