Geeetech I3 pro 8mm Acrylic X, Y pully guide

***** Please see below update *****
When the X, Y pulley are not centered correctly, the belt slowly moves to one side till it gets stuck and then your lose your print.

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Update:

I have found the ball bearing that fits the printer belt.
http://www.banggood.com/GT2-Synchronous-Wheel-3MM-Without-Teeth-For-3D-Printer-Accessories-p-1013245.html?p=9R09143243493201512Z
You will need 2 screws from bag #27 ( M3x20mm ), and 2 Hex Lock nut from bag #13 ( Hex Socket nuts M3 ). There is not en