where is you second tank located at?

I just got an 81 F250 with a 351 m dual tanks.
It has only one filler door for the frist tank .
where is my second tankand how do i fill it????

It should be about 3 feet to the right od the first filler door when your facing it. Maybe someone moved it?

Or the box was replaced with one from a single tank truck, so it is missing the rear fill door.

the second tank is in between the rear wheel well and the front of the bed.
it sets between the frame rail and out side of the bed with the filler neck in the left side of the well.
( can you say ford pinto :)

Um, that is NOT a factory tank. IMO, you should remove it IMMEDIATELY!

I had an '87 and mine was between the rear end and the spare inside the frame. The secondary filler was behind the wheel well and the primary was in front. Those saddle tanks like you describe were discontinued in the 70's. Are you sure the previous owner was'nt running moonshine?

i'm going to. in a week and a $100.00

the guy i bought it from as the the paper work on it and th dealer is the one who put it in. The date on the paper work say 1981 june 12 wednesday.

I'm not sure when but installing gas tanks outside of the frame members was made illegal in the early '80s. Is there a pipe or hose of any kind connecting the tanks besides the primary fuel line?

no all it has is the filler tube, vent tube,emissions, and gas line.