76mm diameter idler pulley can be used to correct belt length when changing the supercharger pulleys or harmonic balancer on the 2009-2013 ZR1 LS9 supercharged ZR1 Corvette.Specifically designed to be use with the 2.38" super charger pulley and the stock harmonic balancer. Must use the 82.5" 11 rib serpentine belt to complete upgrade. No Discount