These are the sprocket/rotors used in our Sprocket Brake Kits. They are available in 48T (9.8" diameter) or 50T (10.2" diameter) options. If you are ordering this as a replacement for your existing sprocket/rotor then you must order the same 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 standard 'evo-era' 1.98" center-hole, but center-hole can be bored to your specified dimension for $50 additional charge (contact us directly for this option). 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 sprocket/rotors are counter-BORED.