The left mirror on my 2006 Sportster at 25K would no longer would stay in place. I noted that the mirror glass could be removed by heating the casting with a heat gun. I had to use a setting of 800 deg and a very small pick to get it off. I then used a rotary tool and removed the bottom left socket support (there were three) and then drilled a 3/16" hole through the casting where the support had been. A #10 nut (held in place by a small amount of epoxy glue) and a 5/8" long fine thread stainless steel bolt then provided tension adjustment. Replacing the mirror glass completed the project.