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I shimmed valves on 07 CRF 450R put the cam assembly back in with the cam chain off and I spin the cam sprocket and it won't make a full rotation. It bottoms out. Why does it do this?

Why are you spinning the cam without the chain on? And by bottom out, do you mean the cam lobes are contacting the shim buckets and you cannot turn it farther? If so, that is completely normal. There's no way you should be able to rotate the camshaft by hand. You have the compression of the motor and the force of the valve springs working against you. Put the chain back on, line the timing marks up, and give it a couple rotations using a ratchet on the primary gear and then check your timing again to ensure it is still good. If it is good, put the valve cover back on and fire her up.