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Which hub do you have on the back? Just thought, may be one or two issues with threads on weird/fancy hubs (same way some cogs don't fit on all hubs) but most of the common rebranded Formula hubs/Surly hubs etc are fine.
Edit: or maybe that applies to lockrings... my fixed knowledge is fading fast...
For more proof/evidence check this thread from like 5 years ago: https://www.lfgss.com/comments/6994957/
That's not quite right, the thread is the same (for a fixed cog and freewheel). The freewheel will screw on and about 2/3 of the threads will engage and be more than enough for every day use.
@c0gsucker - you should be fine with the hub you've got already, just stick a bunch of grease on the threads and use a freewheel that isn't made of cheese.