If you have purchased these type of mirrors you probably noticed when installing they just don't tighten enough to keep from moving. That or you broke the bolt trying to get them tight. Well after trying different things this is what I came up with. I used a 3/4 section of copper pipe to make shims.
By cutting it lengthwise in thirds and using just two peaces it fit inside my handle bar perfectly and was very easy to tighten down. I laid the shims on the mirror assembly then inserted the three peaces together. It was a little tight so I tapped the copper shims in carefully with a pair of pliers, making sure the three peaces stayed together. After seating it all the way I put it all together for positioning. First I positioned it where I wanted then removed the bolt and applied thread lock. Then put it together again. It's very tight now and I seriously doubt it will come loose again. Hope this helps out, a simple but effective fix.