Solidworks wasn't happy with the STEP file I converted (I can't remember where I got it from though, so the files at the link above might be fine), so I spent some time last year fixing the surfaces it was complaining about. If anyone else is working in solidworks and had that problem before, I believe the file here is fixed:
https://workbench.grabcad.com/workbe...LC/link/332758
(you can also download that as a step file but I'm not sure if it'll have the same problem again)