Hmmm - Not so simple:

There's 1.27 and 1.39 diameter hubs, both with 24 tpi left-hand threads.

The JoyTech hub has both splines and threads (for the locking ring). There is no document that I can find that gives the diameter for the JoyTech.

Shimano makes multiple splined hubs for their cogs. There are the 3-tooth Alfine cogs, and others with more splines.

NOBODY that I've seen seems to have a clear description of the hubs, their dimensions, or their cog compatibility. Cambridge Bicycles seems to offer the most cogs that I've seen, but even their site doesn't go into any detail about what is (or isn't) compatible. Sheldon Brown's site seems similarly bereft of good information.

In addition to the incompatible "track" and "single speed" cogs, there are also a host of incompatible coaster-brake, three-speed, internal-gear, and multi-speed cogs.

Considering the voluminous information available for all other aspects of bicycling online, I'm very surprised at the lack of single-speed cog info. Does anyone know of a comprehensive reference?

Thanks - FH