I have a 1991 Cannondale R600 with a threaded Tange Seiki headset. The seal between the top race and the locknut is mis-shaped and needs to be replaced. I can't get the headset adjusted correctly because of this problem. Where can I find just this rubber seal? I could just replace the entire headset, but it's in good condition and requires more work, so I'd prefer not to do that.