The typical expansion plug weighs around 30 grams, and doesn't serve a whole lot of purpose once the stem is tightened. This has always bothered me.

It bothered me to the point that I tried the Tune Gum Gum expansion plug, which is a rubber spacer, a silicone-ish expansion disc, and two alum alloy discs, one threaded. The idea is that the two alum alloy discs squeeze the silicone to get a grip on the inner steer tube walls. I've never found them all that effective -- they can keep a top cap on ok, but the hold isn't all that great for putting tension on the headset bearings when installing a stem.

My idea is to make a lightweight disc of carbon fiber sandwiched around something light in the middle, such as balsa wood, drill a hole in the middle and press in (or epoxy in) an aluminum alloy riv-nut, as frame builders use for water bottles (or used to) with ti or other non-steel frame types. I then would epoxy that into the steer tube.

Riv-nuts are generally for M5 bolts, and steertube top caps are designed around M6, but this can be solved with an M5 bolt with the right head profile.

This plug would be semi-permanent, sort of like star-fangled nuts are, but that doesn't concern me -- my forks are cut to the length I know I want them, and I feel comfortable removing an epoxied piece if I needed to.

Anyone tried something like this? It would save about 20-25 grams, and I think be a more elegant solution than what exists. I know I should be posting this to the weight weenies forum, not bike forums, but who needs another login credential.