When you put the bung in the tank, the center screw is to be tightened until it is secure and flush with the tank. The screw expands the bung as it tightens and seals it completely if you get it tight enough. The silicone tubing just needs to be slid on the copper pipes and they won't come off granted you have at leat 1/2"to 3/4" to go on the pipe. Hope this helped.

Gibbs