Originally Posted by
JohnDThompson
If that's the original wheel, the rear hub is likely metric thread and you won't easily find a fixed cog to fit on that hub.
Ah - that sounds like a problem. Yes, it is the original wheel. Maillard hubs? I think.
Pop the freewheel off and check the hub thread. If an English thread bottom bracket lockring threads on properly, the hub is English thread and will accept a standard track cog as Sheldon describes. If not, you'll want to find a different hub for your fixed gear conversion project. May as well get a proper track hub or flip/flop hub in that case.