Say whatIJ Guess you guys got it figured out later on, but just had to refute this statement anyway. The drive (top) pulley, if it's working right, is going to force the belt to the outer limits no matter what its length is. The shorter the belt, the wider that will force the spring loaded driven (bottom) pulleys, thus increasing the speed. The crossflow fan changeover required the shorter belt when installed in the 1600 series machine. I converted one of these. I'm assuming the 1644, 66,88, 2100 and 2300 series still use the same short belt.