The revolutionary machined design of the XX Cassette uses the same grueling manufacturing techniques that produced the PowerDome road cassette. With X-Dome, eight of the cogs are CNC-machined together out of a single block of billet steel, creating an incredibly lightweight, precise, and strong cassette. It's a process that takes nine hours per cassette! The aluminum large cog can be replaced as needed. You have never seen a cassette that looks like this. The open design aids in mud clearance, giving you cleaner shifting performance and longer component life.