I am in the process of putting together my 77 F-250 4x4 and I wanted to replace the existing fuel lines because one of the big upgrades of this overhaul is adding dual tanks for the thirsty 400.

Now I am into the project and I realize that the new lines I have are great and fit like a champ, but they are 5/16, which is the same as in my SBF F-150, but somebody put in 3/8 lines from the tank to the fuel pump. I am 90% sure the line currently on the truck is homegrown and not factory installed. I never picked up on this before, but before I completely install the new lines I wanted to ask if the 400 trucks came with 3/8 lines and if then need them.... the tanks are all 5/16 at the senders. The rear tank was 5/16 with 5/16 rubber pressed onto the 3/8 solid line.

 

My tank was also 5/16 coming off the tank, but 3/8 at the pump inlet, and 5/16 again going out of the fuel pump. Its a rather strange combo.

 

It depends on what you have done to the 400. For a stock 400 (or even a mild cam, intake and carb) the 5/16" line will be fine. If you have a double pumper Holley on a high rise X-ram with 2" primary tube headers, etc, you may want to consider the 3/8" line. You can never have too much volume. If you plan to upgrade later, go ahead and put the bigger line on it. People tend to overbuild the fuel system because they think they need it, but in reality, the factory engineers were pretty smart guys and knew what they were doing.

 

Agree 5/16" will generally work but 3/8" has the advantage in hot weather even in a mild combo if you are running a stock style mechanical fuel pump.

Less restriction on the suction side means less chance of vapor lock. With today's gas that likes to boil, that's an important consideration.

I'd go all 3/8" personally. Also dual tanks are the OE solution for added range... but a big 38 gallon rear single tank is simpler and readily available.

 

61steven: I am pretty sure my F-150 is like that where the 5/16 line expands at the end where the rubber line connects it to the pump, but is 5/16 back at the tanks.
earthquake68: My 400 is and will stay pretty stock, but for higher compression pistons and an intake that it came with. It uses a 4150 700 cdm carb, so it is pretty stock type setup.
I guess I will just run what I have because this is really a truck truck that is used for work stuff.
I was just wondering if 3/8 was needed. I know that in road racing some people use not only 3/8 but also 1/2" fuel lines and it helps with supply if you have suction type pump and a big carb and are running at WOT all day, but I thought most engines are fine with 5/16 for stock applications. I also know that some people prefer to stick with 5/16 line because of space/clearance and then use an additional in-line fuel pump to meet demand. I feel like 5/16 will be fine, but as you said "the factory engineers were pretty smart guys and knew what they were doing." and I want to go that way.

 

5/16th is appropriate for that application