This ball caster can be used as a replacement 3pi robot part or as a general-purpose ball caster. When mounted through a hole in a chassis that uses our micro metal gearmotors and our 32×7mm wheels, as on the 3pi, this ball caster is just the right height to keep the chassis level. The hole for this ball caster in the 3pi chassis has a diameter of 600 mils (0.6") and has a side that is flattened by 30 mils.
Replacing the 3pi Ball Caster：
To replace the ball caster on your 3pi, you will need to unscrew the rear battery holder (pull off the LCD and remove the batteries to access the screw) and gently bend the holder far enough out of the way to pull out the existing ball caster and slip in the new one. The battery holder is soldered to the 3pi PCB chassis on one side, so you will not be able to fully remove the battery holder. Take care not to lose the plastic spacer that is underneath the holder.