Threaded freewheel cogs can be as cheap as you like. https://www.charliethebikemonger.com/freewheels-screw-on-224-c.asp
Btw, are you sure you don't want to make it a fixed gear conversion instead of a single speed conversion?
Thanks, I bought a super cheap one from sjscycles just now. Have every part but a brake, a shim for my bars and my grips now.
I'm 98% certain I want to make it a single speed rather than fixed, yes. Why do you ask?
@LightningPesto started
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Threaded freewheel cogs can be as cheap as you like.
https://www.charliethebikemonger.com/freewheels-screw-on-224-c.asp
Btw, are you sure you don't want to make it a fixed gear conversion instead of a single speed conversion?