Don't know about screw top caps. I suppose if they are plastic or aluminum it would not matter, but any ferrous material will have an impact of the fluxgate compass. I had stored an umbrella and it took me a while to figure out why the autopilot would suddenly turn 90 degrees to port when headed due south. Worked perfectly otherwise. When I finally decided to remove the fluxgate to have it fixed I saw the umbrella. I should say I saw the light ! I removed the darn thing and also replaced the plastic coated coat hangers with solid plastic ones. Did not need to have the compass fixed and it has been working flawlessly for the last 13 years since.