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gthomson looks like the image of the keyed washer didn't come through in my previous post. You need to have a washer with a key in it in place of that light mount. See below. Notice that little nub at the inside top of the washer. If you don't have that the headset will become loose over time. Your old light mount probably had a key in it for this reason. If your headset is adjusted well right now (no binding from being too tight or rocking back and forth) it would be enough for a little bit of riding but plan on trying this again with a second wrench and a new washer once acquired.
