Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 22 Post(s)
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Think of cones and cups.
Most bicycle bearings are "angular bearings" using a conical part, and a cup shaped part with the balls trapped between. Now look at your headset. There's only one crown race (probably still on the fork). It has to mate with a cup, usually with an extending skirt coming around the side to enclose it.
Like wise, there's only one threaded bearing race for your fork. Depending on the headset, it might be a cone or a cup (both designs exist), but it needs to mate with a top of frame bearing of the opposite design. If the threaded race is a cone, there's usually a short skirt around the outside which will fir around the upper frame cup as a weather shield.
I hope that helped.
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WARNING, I'm from New York. Thin skinned people should maintain safe distance