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I just installed a CM aluminum RD flatbed on my ‘97 F350 and am considering trying to use the standard single fuel filler neck to fill both tanks by installing a T in the filler hose to the main tank so once it’s full fuel will flow to the rear tank (as I fill up) and I don’t need to cut in a second filler neck. Is there some legitimate reason why this is a a bad idea? Seems like this would work, but interested in pros and cons if you have thoughts…
 

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You would need to ensure that there is adequate venting when filling and that the tee should probably be more of a wye, otherwise it may just back up on you. Cheers!
 

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'79 F350 xlt extra cab 7.3 auto dually. Stock. 240,000. Bought new.
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I know this is an old post, but the answer is it would require welding in fittings at or near the lowest points of the tanks. The tanks need to be connected by braided hose and protected from broken drive shafts and such. Each tank should also have ¼ turn shut off valves so that if one leaks, you can stop the other from leaking out too. Then, you will only be able fill both tanks as fast as the fuel can pass through whatever size hose you use to connect the two, and the limitations of the ventilation. I'm sure there may be a better way? That's how some big trucks are set up, but not for fueling. For equalization as you use fuel. (One sending unit, two tanks)
I'm sure it could look good with two filler doors?
 

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Roadway Express (used to work in one of their shops back in the late 80's) used to run a 3" crossover hose from the left to right fuel tanks that would allow high speed diesel pumps to fuel both tanks from one side. There was a 3" spud at the bottom of both tanks facing inward and a hose that ran straight across between the tanks and below the driveshafts on some of them. There were no shutoffs on this large of a hose at the time. This made for a big mess in the event of a driveshaft failure or running off the road where the crossover was damaged or completely torn off the truck. As Sagelike said, you would have to run a hose from low on one tank to low on the other tank and fill one tank with the fuel transferring to the other tank. You would have to use a large enough hose to pass fuel between the tanks as fast as it runs into the "primary" tank that your fill neck would be connected to.

It would probably be easier to install and use both fill necks, certainly would be safer.

Dave / Believer45
 

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Perhaps a better solution for you would be to replace the rear tank with a Bronco tank (approximately the same capacity as the current 2 tank setup. Then just forget about the forward tank.
 

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Perhaps a better solution for you would be to replace the rear tank with a Bronco tank (approximately the same capacity as the current 2 tank setup. Then just forget about the forward tank.
Who needs 38 gallons of fuel? Just replace it with a battery! That solves all the problems!

Oh wait... sorry, reality just kicked in 😋
 

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...a T in the filler hose to the main tank so once it’s full fuel will flow to the rear tank (as I fill up) and I don’t need to cut in a second filler neck.
Yes, it will work fine for filling, and the 2 otherwise-separate tanks will continue to work as they would with separate filler necks. As long as the split is several inches lower than the bottom of the pump nozzle during filling, it shouldn't matter what shape it is (T or Y) because the liquid can't stand on either shape. It will fill both tanks (eventually), & vent during filling as normal because there's still only 1 pump nozzle dumping fuel in, so the same flow rate of air coming out as if the neck went to only 1 tank.
 
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