Well I had laid it out on the pattern and all was well but I was still not sure as to the cut out being right and I started thinking how I could check it again an be more accurate. I measured and transferred that to the pattern and it still came out right. Then I thought about one of my laser levels that shot a beam straight up so I blocked the wheels on the plane and drew a line where the cowling should be on the exhaust pipe and marked the center of the line and the center of the pipe right and left turn on the laser and centered on the marks, then taped the laser level in place and removed the muffler. After installing the cowling I turned on the laser and the mark fell right in line with my layout mark from the pattern. So I removed the cowling and using my jig saw cut out the opening leaving 3/8" gap around the exhaust pipe. Then re-installed the cowling to see it in place.
