Brief Description: Fuel Filler Righ to Upper Wing Skin Fit
Here is one solution to making a seamless fit of the fuel filler ring to the wing top skin. I made a plywood ring the thickness of the distance from the top thread to the ring flange and then inserted it under the skin and on top of the fuel tank. When you tighten the ring it pulls the skin and tank together and makes a flush fit for the ring.
The shim is split to allow it to be threaded underneath the skin and the top inner edge is beveled to fit the under side of the ring.