Just installed my campy hidden headset onto my new frame, however, I discovered that there is a gap of about 1mm between the top cup of the headset (the one that sits below the stem) and the headtube. So i'm wondering if this is normal for a campy hidden headset. The only concern i have is that the gap may allow water or dust to go in and cause further problem

thanks

Thats pretty normal. I don't know about 1mm, but there is a slight gap in mine as well. The bearings aren't completely sealed on those sets, so you should be overhauling them with some frequency.

I found this problem, and got suggested to put a thin o-ring in to fill the gap. This fixed the problem (install it with some grease to keep things smooth), my next frame had a headset with a little rubber wiper on it that closed the gap without adding noticeable drag (the o-ring added some drag I could feel).

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Ride lightly!

that gap is normal. but you should have a rubber gasket installed. i'm suprised it doesn't come with the headset.

On my bike the gap is more like 0.5mm or less. The bearing is sealed pretty well with a rubber lip. Paste a liberal amount of thick grease over the seal and you're good to go. No problem with dirt or water intrusion.