Originally Posted by
Dave Mayer
Once the headset bearings are properly loaded, and the stem bolts secured, the expander can in fact be removed. It does NOT provide structural integrity benefit to the system. The expander and top cap ONLY exists to allow the headset bearings to be properly loaded.
This is highly debatable. If you've seen enough old carbon parts, you'll see deformation sooner or later. I've seen carbon steerers with reduced diameter wherever the stem was clamped, with a bulge where the hole in the stem at the back of its extension was. Seen the same on bars. It's a pretty illustrative rationale for the 5mm above precaution.
A well-designed expander plug should support the steerer properly against crushing force, but many are sub-par. I make sure to use good ones so I can cut the steerer level with the top of the stem. Then I countersink them and modify the top cap if necessary to sit flush.