Product details:

Wolf Tooth 42T Giant Cog - Now you can take advantage of similar gearing of the far more expensive XX1 setup. The Giant Cog is a 42T cog that gives your ten speed cassette a much wider gear range than the standard 11-36 cassette. Available in SRAM and Shimano, the Giant Cog works with XT, XTR and 1030, 1050 and 1070 cassettes. It requires the removal of the 17T cog and spacer.