"Your bigger problem (in terms of going cheap) is going to be that if you have a french bike, chances are your rear hub has a french thread freewheel and won't take a standard track cog."
Most all french bikes imported to the US during the bike boom used English threading for the freewheel, oddly enough, given French stubborness on other parts. Of the several French bikes I have, only the 1967 PX10 is threaded French on the freewheel. While you do find French freewheels on occasion, esp. on higher level and rarer French bikes, most French bikes in the US have English threading on the freewheel and will take a track cog no problem.
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