OK, I have a '56 Plymouth (wagon) that I am changing to fuel injection, the old original tank has not had fuel in it for many years. I took the plug out today to see what kind of gasket was used. I used a 1/2 drive extension for the double hex plug. NO gasket, I am sure there is supposed to be one, it just had disintegrated. The OD of the threads is 7/8 inches. I think any nylon or copper gasket would work.