Question: Do I trim the open end of the exhaust outlets to fit over the factory installed "circles" that are attached to the rear of the fuselage? Answer: Yes. The circles are 3-1/8 inches (79 mm) in diameter. When trimming the open end of the exhaust outlets to fit over these circles, trim any flashing away in small steps. If you over do it, the diameter of the exhaust cones will end up being 3 inches in diameter or less and will not fit over the factory installed mounting circles. If you do over  trim the exhaust outlets the easiest way to resolve this is use a magic marker to black out the edge of the factory circles and then cut new circles to match the inside diameter of your trimmed down exhaust outlets. Once you have cut new smaller circles glue the new circles in place on top of the factory installed circles so that they are centered within the circumference of the factory circles.