Let's start with the center hole. A screw started from the outside of the cap, through the assembled stopper holds everything together and maintains an air tight tank. The weight is called a clunk and it is on the end of the fuel pick up line and terminates near, but not touching the end of the tank. The purpose of the klunk is to hold the fuel line at the low point of the tank regardless of attitude (inverted, knife edge, or upright flight).

Also, if your fuel line will be easily accessible, you will only require 2 lines. One for the pick up and the other as vent which connects to the muffler. The vent line in the tank is short and faces upward, above the fuel. Sometimes there is a little "bubble" designed into the top of the tank to accommodate the vent line.

I would recommend searching the internet and looking for an illustration of the fuel tank assemblies.

Good Luck,

DaveB