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No idea mine have have always just popped off but there is no reason to glue one on as they aren't really known for falling off so probably just a bit of grime and possibly corrosion making it so hard to remove. Have you tried lots of lube? If not you can use corrosive I'll try and find the brand name of the stuff. I do have a proprietary carbon compression bung in my principia fork that i don't think Is ever coming out. CK looks great by the way
Regular 1 1/8 th on that one I've had loads of fsa headsets and they just don't last cane creek are ok but Ck, acros or hope are the best for durability and Ck have the added bling factor and I've got one in my principia at the moment