0frollard 11 years ago As small as machinably possible. You want to have it as close as possible WITH CONSIDERATION for changes in size because of temperature change, and wobble (if its not 100% exactly right) you get exploded bits. Closer magnets and coils = better magnetic flux continuity = better efficiency. If you can machine it to a tolerance of less than a millimeter so much the better, but you'd better be darn sure about it holding up to strain and not contacting. Tips to get it closer: If its a disk-brake style on a flat plate of magnets/coils, get your plates as flat and smooth as possible so they can be close. If it is a drum-brake style 'cage' of magnets around a stator, then use specialty rounded magnets to make a true inside diameter circle. If thats not an option (too expensive) use more, smaller magnets in clusters. This has the same/more effective magnet area, but more sides on the proverbial polygon to make a rounder surface.