How can I stop a star-fangled nut from rising up inside the steerer, preventing adequate tension, when I tighten the top cap bolt?
If you're applying that much tension to the bolt, something else is wrong. The pull-out force of a star nut is at least one order of magnitude higher than the preload required on a properly installed headset.
If you're applying that much tension to the bolt, something else is wrong. The pull-out force of a star nut is at least one order of magnitude higher than the preload required on a properly installed headset.