Engine oil filler cap screws into this hole in the rocker cover. I noticed the threads in the hole have three colours red which is the paint, silver which is the bare metal and also another type of dark n dirty but solid surface that doesn’t wipe off. What is it, does it need removed and if so how can I remove it? You see less of the stuff from top but there’s more inbetween the threads which the camera could only get a blurred pic off.
Thanks