I had this same problem recently when fitting a new headset - the top cup wouldn't screw on but the locking nut would. most likely the problem is in the thread pitch. does your new headset give any indication that it is italian threaded? english threads have a 60 degree angle whereas italian have 55 degree.
I had this same problem recently when fitting a new headset - the top cup wouldn't screw on but the locking nut would. most likely the problem is in the thread pitch. does your new headset give any indication that it is italian threaded? english threads have a 60 degree angle whereas italian have 55 degree.