Snipped from Fixed Gear Mailing List:

jared stephens wrote:
>it seems like surly goes very well with surly.
>wouldn't it make sense to buy a cog made by the same
>folks who made your hub?

You would think so wouldn't you?

As it turns out, however, the Surly sprockets have an unusually poor
"track" record, and I've stopped featuring them for this reason.

The problem is that they're too thin, so there's not enough thread
engagement. The chart at http://sheldonbrown.com/chainline lists a
buncha different fixed gear sprockets and their vital dimensions.
You can see that Surly is among the worst in this regard.

Generally, I'm a huge fan of Surly products, but the one exception is
their track sprockets, which I strongly advise against.

See: http://sheldonbrown.com/harris/fixed.html#sprockets

Sheldon "Size Matters" Brown
Evanston, Illinois