I am also upgrading the belts. I am fond of Gates fiberglass reinforced belts and I wanted to change the idler from a smooth to a toothed pulley because that is the side of the belt that rides on the belt. I will never know why all these printers ship with smooth pulleys when they ride on the toothed side of the belt. Toothed idler pulleys are not hard to come by and they are the same price everywhere as the smooth pulleys. I am actually performing this same upgrade on both axes on all 3 of my printers..