Joe, The cap / rubber stopper can just be used as is, sometimes I'll coat it with
some light grease etc. just to ease it into the tank. As you tighten the center screw
it will squeeze the rubber stopper to make the seal on the tank and tubing. I have
never had one leak yet. It's a good idea to use tubing clamps or twist some wire
on the fittings/tubing on the inside of the tank so they stay on, I have done it with
and without with no problems to date..
The only place I use silicone is on the outside of the stopper cap/tamk and the
firewall to seal and hold the frt. of the tk.