I am building a Rostock 3d printer and wanted smaller pulley to get a higher definition. Smaller pulley decrease speed but quality should be improved.This is basically the same Carriage made by Johann (http://www.thingiverse.com/thing:21004/#instructions) but I added few variables to make it more flexible. You can get the SCAD file and add your custom pulley size for different size. I made 3 STL files for GT2 pulley with 16, 20 and 40 teeths. For the 16/20 teeths, you can use this flanged ball bearing on the Idler End: Flanged 8mm x 12mm x 3.5mm http://www.aliexpress.com/item/Flanged-8mm-x-12mm-x-3-5mm-Metal-Sealed-Deep-Groove-Ball-Bearing-10pcs/1341229739.html