Dr. Pulley 16x13 Sliding Roller Weights for 50cc 4-stroke QMB139 engines. Please select weight from drop-down menu. The following sizes now come in Black Pearl material: 5g, 6g, 6.5g, 8g. Please see photo gallery for an example. Traditionally, gear ratio is changed by the up-and-down rolling movement of the roller weights in the ramp of the Movable Drive Pulley (MDF). Now the innovative Dr. Pulley Sliding Roller Weights slide up-and-down the ramp to change the gear ratio. This unique sliding movement (not rolling) provides exclusive advantages to increase CVT performance. With Dr. Pulley, no abnormal wear will occur even when running in high gear.Please see our Warranty & Returns Policy. Size: 16x13Set of 6Available in 3 Gram Through 11 Gram Weights, Including Half Gram WeightsBroadens your range of gear ratiosIncreases acceleration and re-acceleration propertiesIncreases top end speedLasts longer than standard roller weightsU.S. Patent Number: 7,276,004Please Note: Weight Color May Vary