Metal valve stems come either with a rubber grommet (regular pass. cats & trucks), or a simple silicone washer inside to seal to the rim. Campagnolos have a small 1/2" hole, so to run such stems, one drills out the rim, or uses the ones with the washer inside the rim. I've seen motorcycle stems used (also metal) and there's one stem sold for rice-rockets that is close-to-flush with the rim, and uses a tiny pin-spanner wrench to untighten the cap. .Dunno how well it works keeping air in the tires, but it sure keeps kids fron stealing the caps for their bicycles!
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