Since people have been asking about it, here it is: All you are doing is twisting the commutator relative to the rest of the armature. When you advance the timing on the motor, you gain top speed (in the forward direction, while losing speed in the reverse direction), at the expense of a little bit of torque. There are limits to how much advance you can get away with before the motor starts running poorly. Stick with what I show in the pictures and you will find the gains to be worthwhile without being too risky.
The first two pictures show the armature with neutral timing. Note how the commutator contact is centered in the gap between the poles of the armature.
The next two show the timing advanced. I try to twist the commutator so that the contact is even with the edge of the pole.
To (gently) twist the commutator, I just grasp it in my fingers, while holding the pinion gear in my other hand. It is possible (and advisable) to do the timing adjustment without removing the armature from the can, as pinion removal tends to force the brass spacer at the front of the armature into the armature windings, causing excessive longitudinal play within the motor. Note that there are small nubs on the end of the commutator that hold it in a neutral position. When you twist the commutator, these nubs will break, allowing it to move. Once you have it where youwant it, you could use a drop of CA glue or the like to hold it in place (towards the windings), though I don't bother, since the rotational force of the motor is primarily on the armature poles, and the commutator is just along for the ride.
Last thing to note is that you create a bit of slack in the windings by doing this, so I tend to try to pull them tight at the contacts, as shown in the bottom picture.
Threaded Mode
