Summary I had some issues alining the belt with the idler, so the belt was falling from the bearing and thus wearing out. Thanks to Aldric Negrier http://www.instructables.com/id/Mega-Prusa-i3-Rework-8-x-Build-Volume/#step1, who suggested me to take a look on this: http://www.thingiverse.com/thing:289676/#files - made originally by juliohcd (for 624zz bearing), I have remade this to fit the 608 bearing used on Mega Prusa Y idler. I deffinitely wanted to use the y belt idler provided by Aldric Negrier because on my printer every parts are mounted and didn't wanted to unmount the rods on the idler side. Please read the instructions for optimal print of this parts!!!! Instructions Printed in PLA, 0.2 layer hight, 2 shells/0.8mm - Cura (use this if you have 0.4 nozzle like me - the parts are made for this nozzle size and 2 shells), fill 100%. You eventually need to clean the surfaces with a rasp and make some small chamfers. I did not left any clearance between the two housings so that you don't need glue for keeping them together, push hard and be carefull to not brake the parts. Also, once fitted together, you have to push the arms of the idler apart gently so that the bearing+housing will fit between the two bearing brims of the idler. Mounting order: 608 bearing -> y belt guide small 608 -> y belt guide big 608 -> y idler