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Do you even Fixxer? The conversion replaces the freehub rotor on common Shimano rear hubs, using the large hollow central bolt for attachment and the splines on the hubshell for drive.
A Specialized/Hed with just 1.37"x24 threads on the hub is for a screw-on multiple freewheel. You can get adapters to move the chainline to suit fixed use, but the adapter unscrews off the hub when you try to brake, so it's still a suicide hub even though the adapter has lockring threads.
Only a small number of early Hed3 wheels work with a Fixxer. Specialized don't, nor do later Heds with their own design cassette rotor. Pro clincher disc with the FH-6600 or FH-6700 based hub works. At some point I'm going to hassle Walker Brothers
about their Wideboy disc, they promised a track one for 2016 but there's no sign of it, and I really want bolt on sprockets anyway