[OrrieG]

If you are running conventional carb with stock engine mounted fuel pump you need a vent. Like Cal said without it and an unvented cap you get a vaccum in the tank. Also no pressure relief when filling. I don't know anything about modern injected engines with the fuel pumps in the engines. My after market tank came with a threaded hole for attaching a 3/8" hose fitting. I a using the stock filler relocated to the bed side so that is where the vent will go. You can get aftermarket fillers from marine suppliers that have the vent line configuration too. Some guys just run the hose higher than the tank and put in a check valve. Search some of the build threads using "gas tank" as a search word and you will find an bunch of different ideas with manf. info or links to suppliers.