Reading back through my post I see I forgot to mention, yes the screw that is the "axle" for the cam has a shoulder, and no there is no lock washerer (nor is one needed) if you have the wrong washer in there it is to thick and the cam gets pinched (locks up) before the shoulder comes to bear, and you can't get the nut tight enough to hold. I accidently put mine together that way tonight and it did exactly as yours does, had to take it back apart and realy look at it before I figured it out. It is easy enough to tell which washer is thick just by comparing them, don't worry you won't need a micrometer or anything to tell
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Again, good luck and let us know if it works or what you find.
Dan