Dr. Pulley 16x13 Sliding Roller Weights 6.5 Gram Dr. Pulley 16x13 Sliding Roller Weights for 50cc 4-stroke QMB139 engines. Please select weight from drop-down menu. 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: 16x13 Set of 6 Choose From 3 Gram through 11 Gram Weights, Including 1/2 Gram Increments Broadens your range of gear ratios Increases acceleration and re-acceleration properties Increases top end speed Lasts longer than standard roller weights U.S. Patent Number: 7,276,004