These pulleys are NOT compatible with M5 bolts. All types of M5 bolt heads and washers are larger than the bearing in this idler, which causes the bolt/ washer to seat on the pulley, not the bearing. This causes the idler to seize up. Robotdigg's official response to this problem is to "go have some custom washers machined". Ya I don't think so . Very disappointed in Robotdigg.