vented gas tanks

I am going to use a gas tank in my roadster pick-up, similar to the one in the picture. My question is this---I know that I need a vented cap that will allow air into the tank so that as the gasoline is burned, I won't pull a vacuum in the tank. The other side of the coin is that as the car sets in the hot sun, gasoline vapours in the tank will expand and have to go somewhere. I cant really have a vent tube in the tank, as there is nowhere for it to go. and I can't just have a vent "hole" because as I drive the car I don't want fuel sloshing out of the tank into the back of my truck.---Does anyone have a solution for this"

A couple of thoughts just to start the thinking process:

1. From your cap height to the gas level in a full tank is a small vertical dimension. Will the fuel "slosh" out of a vented cap? Does is sloch out now with a full tank? A lawnmower has a vented cap that doesnt allow fuel to slosh out so is there an equivalent, decent-looking cap for this application?
2. You could tap a vent line into the spigot just under the cap. However, to be effective, there would have to be a vertical vent tube rising up from that port....not really good looking.
3. A vent that exited the tank at the highest point and ran parallel to the fuel line would work but the exit would have to be above the tank full level.
4. The 2nd easiest solution is to extend the cap spigot a bit and use a vented cap...the amount of extension is determined by ???
5. The easiest solution is to use a vented cap and to not fill the tank past some level that causes the problem.....makes an already small tank smaller

Just random thoughts.

mike in tucson

Using some existing technology in lawnmower gas caps, you could probably modify your existing cap so it is leakless and vented, how you do it is you have to drill small holes in the cap, about 4, then make a gasket type thing from some soft rubber or leather ( old mowers did use leather as the gasket ) then, in the same places as the holes cut small x's, and put it in the cap. It will allow air to enter the cap, but when the gas sloshes up at it, it will push the small x's closed and no leak. This is how mower caps work and most mowers have no suspension so there a lot bouncier than your roadster pickup. If the cap was stainless, the burrs the drill bit might leave behind, will polish and smooth right out with a buffing wheel. I'v never used this in anything automotive but I have replaced the worn out leather gaskets with my own rubber 1's, cutting the small x's and it works.

I'm going to go down to the Harley Davidson dealer tomorrow and see how they cope with the situation---it seems that motorcycles will have exactly the same issues with their gas tanks.

Brian, I been thru this one a few years back on a customers 84? chevy pickup, after repeated fuel pump failures he had me install a frame mounted electric fuel pump,easy deal right ? well it was till it came back on a tow truck because it had sucked a vacume on the tank. The original mechanical fuel pump had a return line to the tank wich creates a closed loop system thus avoiding the vent problem,(we plugged this line when we installed the electric pump)
You could install one of those style pumps if your using a mechanical pump and run a 1/4 return line back to your tank, or in the case of the customers truck I fashioned a tee off the 5/16 fuel line up by the carb & ran a 1/4 line back to the original return line, we did hafta play with this and wound up installing a # 65 hilborn injection pill in the return line too keep enough fuel preasure to the carb....that was 7 years ago and the trucks still runnin......

You nailed it Denny---thats the one and Dans the guy I ordered it from. The first tank, 10" diameter x 40" long came in 2 weeks ago, all dented to Hell by UPS. Now I'm waiting for the second tank, which has been shipped direct to me by Dans supplier out of Wichita. I ordered it with brushed aluminum straps and a billet vented cap.----They are really nice looking tanks (minus the dents).