Surly Singlespeed cogs Track Lockring
Solid steel singlespeed cogs for use with most singlespeed hubs.
Available in two types: fixed (track style) or freewheel. The fixed cogs screw on to the hub while freewheel cogs fit cassette-style hubs. You'll also probably need spacers with the cassette style cogs (often supplied with the hub), and a lockring for fixed cogs (available separately).
Currently available in 3/32 chain size.