Originally Posted by well biked

In marginal situations, the Park site recommends Loctite #RC609. I don't know if you saw the discussion about a headset problem in the C and V forum recently where an Italian frame was being built up and it was found that the headset that was being used was a JIS headset, which meant the crown race was too large (ID) for the fork and the cups were too small for the headtube (ID), the cups could be pressed in by hand. In that case the consensus was to use an ISO headset. In your situations, if you're confident you're using headsets of the correct standard (ISO, JIS, etc) then you could try the Loctite and see if it holds the parts securely. There can be variation from one "ISO headtube" and another, for example, a range of a couple tenths of a millimeter that's acceptable. It may be that you're experiencing a sort of "worst case scenario" within the various ranges-