Originally Posted by Mich48041
It is a bad idea to tap the plastic unless it is firmly clamped to the aluminum. Any space at all between the two materials during tapping will result in the threads being out of alignment. That will prevent the two materials from being drawn together tightly when the screw is tightened.
An option is to not tap the acrylic. Instead, ream out the holes in the acrylic so that a screw slides though the hole without turning.
To comment requires an understanding of the recommended drilling tapping, and finishing processes.
The threads are not left in the window when completed. The window is only tapped during the tapping process to provide a means of aligning the tap so that the threads are not cut at an improper angle. The holes get final drilled later to provide for a slip fit for the screws
Also, there is a gap on some of the holes common between the roll bar and the rear window so the window will never lay flush against the roll bar at all hole locations.