Occasionally the little brass screws for the thumb covers don’t like to come out easily. If you have a broken screw, here is what has to happen to repair it to the original size. Sometimes you can drill out the broken screw and HeliCoil® the threads to the 2-56 size; however if the holes are beyond repair then you grind out the area and fill with new material by welding. Then using a template drill new holes and tap for the 2-56 threads. Note, If you have an adapter for an auto pilot, those use #4-40 threads. See the photos below.