Why does the fuel tank fill line have a 1 inch pipe inside a 2 inch pipe.

[Jdog87]

I am trying to relocate the tank fill line for better access. I took the cap assembly off and noticed that there was a 1 inch pipe inside the 2 inch line. What is the purpose of the 1 inch line and do i need it .

 

[Clb]

Inner vs outter vents

 

[franklin2]

When you pour all that fuel in from the station gun, there is a lot of air that needs to escape out of the tank. They have to keep all that air separated from the fuel going in, or it will cause gulping and spitting and the fuel station nozzle will keep cutting off.

Even with that system in place, it's not really that good especially for foamy diesel fuel, so most people take the roll-over valve out of the top of the tank and modify it for a large hose/aux vent. This works much better than the factory system.

 

[icanfixall]

This was a very poor design. Fuel flows down the center and air is SUPPOSED to flow up around this center tube. Not really a good design. Most of us do a fill line modification called ... Harpooning the tanks... The tank is removed. The top of the tank and a rollover vent. That is removed and a larger line is added to the hole and ends up in a weld nipple near the top of the fill neck. The idea behind the rollover valve is if the rig ever rolls the fuel wont run out because the rollover valve checks it.
If you can reach in to fill line you can open up the tiny air slot near the top of the fill nozzle. A dremal grinder or hammer and screwdriver will open it up. Try to double the size of the tiny slit...

 

[typ4]

Usually the inner hose droops inside the tank and that is what causes the maddening slow fillup.