KMC Sprocket for SHIMANO Narrow 21T E-Bike Hardened chrome-molybdenum steel For Shimano Maximum performance that saves battery life Perfectly matched to KMC chains Recommended chain: e1EPT Item no.: BSRS5021 Size: 1/2 X 3/32 Color: Black 3500km+: It lasts. And it lasts. The 3500 HP+ drivetrain is the perfect solution for demanding drivers. The combination of our e1 EPT chain and narrow sprockets ensures a guaranteed mileage of 3500 kilometers and more.