These are the best idler pulleys we have come across. They have the same 20 teeth GT2 design as our 20 tooth pulleys but combined with two bearings. We recommend these over the standard delrin pulleys or the use of bearings with washers.
They are guaranteed to eliminate any belt wander and to keep your belts exactly where you want them. They are perfect to use with a M4 bolt and are a direct replacement on most 3d printers.