The old MK0 picture shows what looks like a quarter turn gas cap on the bike. This bike has what I suspect is a replacement MK2 gas tank because it has no green paint underneath the yellow and a fine thread MK2 plastic gas cap. I also suspect the original gas tank, wheels and muffler came from the 4a Maico bike which also shows a quarter turn gas cap. Since the fork was tweaked about 5 degrees from a misadventure it is not difficult to believe that the gas tank got damaged and was replaced at some point.
I am not sure why there are disk grinder marks all around the gas cap filler neck. I decided to look into a period correct replacement cap and came up with a late 1950’s Harley cap and filler bung that looks real close.
I took careful measurements and figured out what size hole saw to use. I turned down one of my old fine thread gas caps that I have been using as a painting mask to fit closely inside the hole saw. I also drilled a quarter inch pilot hole in the cap to help guide the hole saw. I screwed the modified cap to the filler neck.
Harley filler neck test fit.
Ready to be brazed.