Originally Posted by
RookDroid
I can't replace the bearings as shimano no longer distributes them.
Yes, that's unfortunate as it was one of the best threaded headsets ever made. I still have one on one of my bikes.
As a test, remove your current cartridges and see if there is any noticable play between the inner and outer races. If the play is noticable, the bearings are worn out. At that point, your only choice is a new headset or finding an NOS HP-6500 and just using the bearing cartridges.