After building a Prusa i3 rework by eMotion-Tech, I found the x axis idler block to be weak, and the printer in desperate need of a way to adjust belt tension. It uses two bearing that were salvaged from a hard drive and a 5mm pin cut down to be just short enough to fit inside the pocket. A 5mm bolt will slide into the bearing pocket and through the outer block where a nut can be fitted and used to adjust the tension on the belt. This is a direct bolt in swap, you shouldn't have to modify the belt in any way. I am using mechanical endstops, so it does not have provisions for the optical endstops like the part it replaces.