A compression ring? Is that the plastic piece that holds in my fork when i drop the stem and brakes? I have to tap out the fork with a mallet every time.
I think i'll get a star nut ):
No plug, as in to a plug your drain or your mouth when you talk nonsense here.... anyways think the compression plug as a top cap but with integrated star nut functionality; as when it's screw is tightened, it's wedges expand upon the steerer tube, holding in place, just as a starnut would. From your point of understanding I doubt you've ever owned a bike whose fork has a carbon steerer tube.
What material is the steerer?
If it's steel or aluminum use a star nut. I just threaded the top cap bolt in a little and drove it down with a mallet.
If the steerer is carbon fiber, you must use a compression plug.