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You just get a fixed/fixed hub and this: http://broakland.com/dixie.html
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is 116mm close enough?
plenty of these have disc mounts and threads for standard freewheel.
http://www.tartybikes.co.uk/116mm_fixed_hubs/c62.htmlWhat frame do you have?
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more 120mm singlespeed disc hub stuff:
If you can find a old shimano 5 speed freehub body (but they are rare now), you can combine it with a standard/older shimano rearhub with a 10mm axle ala:
I made one 3 years ago still working! -> https://www.lfgss.com/conversations/246792/
does anyone know of a 120mm spaced single speed disc hub? (does one even exist?)