Surly Track Cog 1/8 X 18t Silver. Surly track cogs are threaded the same as a BMX freewheel (1.375 x 24 tpi right-hand threads). This means that they will fit on single-speed rear hubs as well as BMX freewheel hubs but this application is not recommended. Track cogs are meant to work on track hubs only.

415 hardened chromoly steel

1.375" x 24 tpi

Photo for color and material reference only