I ll try to make this as painless as possible, and quick too:
- Take out all shims from your armature, remove brushes and springs.
- Put armature in can and screw endbell on
- Put a pinion gear on motor
Spin manually the armature, as it stops it will center itself between the magnetic fields, leaving play on both sides (endbell side and can side)
Shim one end (where you seem to have most play) and repeat process, adding shims on both ends until you have very little play left (ie you can feel some play but do not see it).
As you spin your armature, it will recenter itself each time.
Once done, This means your magnets will not "pull" or "push" the armature, giving you better efficiency..
hope i was clear and hope it helps..