The pulley is made of aluminum alloy, so that it is very firm, lightweight and durable. Anti-rust and corrosion resistant. It run on smooth ball bearings, and it's a necessary accessory for your bicycle. There are four different colors for you to choose from, to meet different needs. Fits for almost all mountain bike of 11T. Hope you will enjoy your cycling. Have a nice day. Features: 1. Made of CNC aluminum alloy, lightweight and durable. 2. Anti rust and corrosion resistant anodized. 3. These pulleys run on smooth ball bearings. 4. A necessary accessory for your bicycle. 5. Fits for almost all mountain bike of 11T. Specifications: Material: Aluminum Alloy Color: Black+Red Cap, Red, Golden, Silver (optional) Weight: approx. 22g Teeth: 11 T Total Length: approx. 4.3cm / 1.69 Rear Derailleur Pulley Screw Hole Size: approx. 5.9mm / 0.23 Package Included: 1 x 2pcs (1 pair) derailleur pulley Note: 1. Please allow 1-3mm error due to manual measurement. Thanks for your understanding. 2. Monitors are not calibrated same, item color displayed in photos may be showing slightly different from the real object. Please in kind prevail. Thanks for your understanding. 3. Please note that the new type and old type of this product will be sent randomly, and make sure you will not mind before ordering.