Another Fuel tank thread - 71 Demon

Oldmanmopar said

up until 72 all fill tube flanges were on the inside of the quarter panel. The screws threaded into the filler tube.

All 72 and later the fill tube flange was on the outside of the quarter and the screws threaded into the quarter panel.

67-70 all had a fill tube vent line and a non-vented cap

71 was the only year the the flange was on the inside of the quarter with a special low locking vented cap

All sport style cars Duster, Demons, and Dart sports, and Barracudas came with 3 attachment screws

All Darts, Valiants, and Scamps came with 4 mounting screws.

They should not be exchange between the 4 and 3 screws because the flange on the quarters are different and will deform the quarter around the fill cap. The reason they are angled different is the quarter on the Dart , Valiant , and scamp are flatter . All the others are wider and bulged so the angle is different. They are also different lengths including Barracuda. But a 70 Duster will fit a barracuda.

So only the 70 fill pipe will correctly fit a 71 but you will have to add the vent pipe because the 70 cap is not vented. A 72 cap will fit a 70 but it will fit very tight.

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Neat and all but none of that really helps the OP or explains any issue he may have.

Maybe the conversion information I mentioned was unnecessary, but my point wasn't about the conversion per se. It was about the fact that the problem that the OP is having with the trunk floor seal shouldn't be a thing. The trunk floor seal itself was the same from at least '70-74, which covers several emissions changes with regard to the tank. The hole in the trunk floor pan didn't move, the same pan is used '67-'76. The filler tube itself didn't really change either, certainly not from '70 to '71. The cap changed, but the tube was still mounted inside the trunk, the angle didn't change, and the hole in the quarter wasn't moved from '70 to '71 (same exact quarter panel was used). Or for '72 for that matter, or even '74 even though those quarters had other changes. While the filler tube was moved from inside to outside of the quarter panel and the mounting of the cap was changed, nothing else regarding the filler tube changed from '71 to '74 for the same model car. Otherwise I couldn't have used a '74 Dart filler tube on my '71 Dart without running into the same issue. And lots of people change those 1 year only filler tubes out for other model years because of the scarcity of the caps.

So my point was that even if the filler hole in the tank moved with that emissions change in '71, nothing else did. Which means the factory and everyone else was able to use the same trunk floor seal despite that change.