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The GC42 cassette cog is so big that when machining the first batch, the machinist thought he made a mistake!  The GC line of cassette extender cogs allow you to modify your 10 speed cassette into a wide range cassette.  To achieve the optimal shifting, the GCs are optimized for Shimano or SRAM cassette. 


  • Compatible with Shimano 10 speed cassettes listed on the GC tech page
  • Compatible with all 10 speed chains
    • Note that you may need a longer chain that you currently have your bike
    • Chain stretch should be less than .5 when installing a new GC
  • Compatible with all 10 speed rear derailleurs except non-clutch type SRAM rear derailleurs model year 2012 and older
  • Can be run in 2x configuration with a long cage rear derailleur

Complementing products:

  • Goatlink - improve the long term shifting when using a Shimano rear derailleur (HIGHLY recommended over the longer b-screw)
  • 16t replacement cog - smooth out those cassette transitions
  • Longer b-screw - necessary with some SRAM GC42 and Shimano GC40 applications.  Necessary on all Shimano GC42 applications if not using the Goatlink

Tech Specs:

  • Installation instructions on the GC tech page.
  • These are specifically designed for use with Shimano cassettes.  The clocking, or rotational orientation, between SRAM and Shimano cassettes is much different so optimal shifting cannot be achieved unless the design is optimized for each separately.
  • Design and manufactured in the Minneapolis, MN USA
  • Material 7075-T6
    • Weight - 86g