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Don't throw away a $400 cassette just because the 42T aluminum sprocket is worn out! The Wolf Tooth GCX sprocket replaces the worn out stock 42T on your XX1 or X01 cassette. Shifting performance matches the stock part.
To remove stock 42T: Gently pry with a screwdriver between 36T and 42T at each pin location. You will need to go around the cassette 2-3 times, moving the cog just a little each time.
To install Wolf Tooth 42T: align large hole (see laser marked dot) with large pin on cassette. Gently squeeze each of the 18 pin locations together with a Channel Lock type pliers until all pins are bottomed out.