Actually, I asked this question a little while ago. Dr. Joe answered with this:
"As to asymmetry in the Chiaphua's. I think that the brushes are symmetric, but I am not going to risk taking the motors apart to find out (by the way, if you do, don't take the armature out of the magnets -- again I have REAL DATA to show that you can lose as much as 10% of the output of the motor by simply taking the armature out of the housing and putting it back in again -- it is a magnitism thing I have been told). "
Seems like that's the best answer we're gonna get.
~Christina