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@gbj_tester
The clearance between a male and female thread allows a nut to move up and down a bolt a small distance without rotating. That's backlash. On a normal threaded fixing, the bolt tension deals with it, but a headset is a lightly preloaded thread, so you can't get rid of the backlash by just tightening one nut (i.e. upper cup) because making it tight enough to eliminate backlash would lock up the bearing.