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In the grand scheme a tap isn't truly needed. It's just one of those tools that make the job at hand easier to deal with. Tapping the hole first relieves the amount of torque you would otherwise place on the mounts. Being that they are plastic, they will snap off and then you have new problem to deal with ;-)
Edge
I just replaced all three on my ultra. We are talking about plastic! I broke the first insert by trying to install it backwards. The slot is NOT for a screw driver. The slot goes into the fairing first. I got the information that tapping is not need from the HD dealer after I went back to buy a replacement.
The installation of these inserts is simple. Use your regular windshield screw and a torx screwdriver. They screw in easy!