Little background, this is for my Ninja 650 project bike. I needed to relocate the stock in tank fuel pump from the Ninja tank somewhere else due to the shock tower and decided to build a subtank to house the pump in the subframe and add some more volume while I'm at it. I am currently building the subtank and have some questions regarding plumbing and venting of the tank so that fill ups are hassle free Here's the CAD drawing And the real thing starting to get formed Stock Keihin fuel pump in custom flange that will be welded to bottom of subtank. Hopefully that gives enough info. Plumbing questions: The main tank I am going to seal off and weld in a bung. I have purchased some -6AN fuel fittings from Summit along with hose ends and hose. My plan is to run a feeder line from the main tank to the subtank. Now if I never hit the subtank on "reserve", it's fill as usual. But say I run the bike completely out of fuel and start to fill it through the tank filler on the main tank. Obviously the fuel is going to fill the main tank faster than the feeder line can fill the subtank so in order to press the tanks, I'll need to be patient at the pump. My question is this. Say I run the feeder line as -6AN which seemed a reasonable size. The fuel draining into the subtank is going to displace the air in the tank and it needs somewhere to go. If I vent the subtank to ambient air, it's going to need to be higher than the top tank level. If I vent it back to the main tank, I'm going to need to run an internal line inside the tank and up to it's highest point? Below is a crude schematic but I think shows what I am trying to accomplish. I could go bigger on the fuel line with the hope that the larger diameter will fill and vent effectively in the same line, but I suspect that it will get annoying and create a bottleneck which will dramatically slow down the filling process. Thoughts?