What keeps the cap on these tanks? There aren't any threads and its too loose for a friction fit.

I only use the smaller tanks but all of mine have a center screw that draws the back piece towards the front and compresses the rubber stopper ,,thats what tightens up the cap

When you put it togeather and tighten the screw in the center of the bung ( cap ) it compresses the rubber bung and seals the tank. Follow the instructions with the tank and you should be OK. You may want to find someone local to give you some assistance.

ENJOY !!! RED

Ok, got it together, thanks!

BTW, be careful when tightening that cap. Too tight and it will split the seam - Not tight enough and it will leak