After mounting it, I realized the carriage was protruding a bit towards the belts, so I have upgraded it. However, the old version is already working (for printing the new one).I am using M5x20mm screws for holding the wheels (which have a 6mm bore) without nuts. Holes are manually threaded by the screw the first time it is used. Same for the arm-holder holes , these are M3 and will be self-threaded using a regular M3 screw. If feels pretty snug but if worried about loosening screws a drop of thread locker or superglue can make that impossible.https://www.youtube.com/watch?v=XOomNgPXUNg&feature=youtu.be