These are the sprocket/rotors used in our Sprocket Brake Kits. They are only available in the 50T (10.2" diameter) option. The 48T sprocket/rotor is no longer available. If you are ordering this as a replacement for your existing sprocket/rotor then you must check the tooth count as the calipers are not interchangeable (because caliper mounting arm length matches sprocket diameter). These are made from hardened stainless steel and measure 0.35" thick. They are supplied with the 'later model' 2.215" center hole, plus a sleeve to convert to 'evolution-era' 1.98" center-hole. If you are replacing an early sprocket/rotor that had countersunk mounting holes, then you will also need to order new mounting hardware (ARP) as these new sprocket/rotors are counter-BORED.