Originally Posted by
FBinNY
I'll bet that you don't have a keyway in the steerer and a keyed washer. Unfortunately these have gone the way of Damascus steel, leaving no reliable way to ensure that the threaded upper cone and locknut don't work themselves loose over time.
Keyways never did that job anyway--have, among other examples a Tange headet with a beefy keyed washer and toothed interface between key washer and cone, and it comes loose all the time. the keyed washer doesn't help because any side to side slip between washer and top nut will tend to unscrew the topnut no matter if the washer is held fixed (
http://www.boltscience.com/pages/josteffect.htm).
The thin upper locknut is a stupid design to begin with, for reasons described at
http://www.boltscience.com/pages/twonuts.htm -- which also describes the answer, stack up lockrings, not washers, so you have more threads "bearing down" than "bearing up."