Looking to convert your standard Shimano HG-splined freehub for single-speed use? The Surly Single Speed Conversion Kit is essentially a bunch of various thickness spacers and a lockring, which combine with whatever cog you decide to use to deliver a stable chain-bearing cog, spaced out to properly create the cleanest chainline possible. Ideally you run a wide base SS specific cog, but not matter what you use, this kit will get you squared away.