Surly Track Cogs. Track Cog Precision CNC Cro-Mo. Wear resistant hardened 4041 material.ISO Threaded &40;1.375x 24 Tpi&41;3/32&34; or 1/8&34; Chain sizes. Note: General Fixed cog check:for correct fixed use the fixed cog should NEVER move on the hub when pedal force is applied in both directions.i.e. if the cog moves then something is wrong. Continued use with a fixed cog that moves will result in damage to the hub.