The drawing in post #5 shows the rubber the wrong way round (i.e. As it was shown in the original factory MO drawing. Post #5 has a note added saying that FT80 needs to flipped around to have the large diameter inside.Isn't the tank rubber FT80 oriented the correct way in post #4, then turned around the wrong way in post #5?
I've always had the larger diameter section on the inside, not the outside.