I work for a Bernina dealer. She has been the tech for over 40 years. Normally, you are told not to adjust the tension on the bobbin screw. However, recently a customer had purchased a pre-owned Bernina 730. She was in for her mastery class and had tension problems. The tech did take the bobbin case and test it with the weight. It still wasn't right and she brought me the little screwdriver and told me to adjust it 1/8 of a turn at a time until it looked right. She also told me the tension screw is set at the factory using 40 weight thread. Is it reasonable to think if you use a heavier or lighter thread it won't need to be adjusted? I won't be afraid to adjust mine when needed. I did so just yesterday ( I have a Bernina 440).