I have often thought how nice an exposed gas tank looks on an authentic Stearman restoration. How did they get it so smooth? Was it one off the shelf after all these years? Was it one of D & S new tanks? Well, I was going to do an exposed tank on a Navy restoration so I thought I had better figure it out. I am sure new old stock tanks exist, but I could not find one. The D & S tank was too expensive — there in lies the rest of the story.
I thought I could pressurize a wrinkled tank and get some of the dents out. I plugged the outlets and let the air flow. (Figs 3&4) I could see the dents starting to expand outward. This is working great? I got the “T” in great out of my mouth when all hell broke loose. The tank blew up like a giant aluminum balloon. All the rivets putted out of the baffles. What a mess! After cutting the end of the tank out and spending hours with the welder, I got it fixed, but what a job. The D & S tank looked really cheap at this point. Got to be a better way.
The jigging shown in the accompanying photos was my solution. (Fig 1) I took 2 x 4’s — straight on the bottom and airfoil shaped on the top (traced off an upper wing rib) connected with all thread. The jig served as a support for the baffles at each location and kept the structure sound.
CAUTION: If you try this at home, make sure you have a 0-15 psi pressure gage hooked up and don’t exceed lOpsi. (Fig 2) That exerts about 20,000 lbs of force to the top and bottom of the tank. Believe me that will move the aluminum. With the tank pressurized, tap around the dents with a soft hammer to encourage the dents to come out.
This will not get the tank baby’s butt smooth, but it will be a lot better than when you started. You could fill the rest of the dents with lightweight filler. I use them right out of the jig. If you are going to cover the tank and avoid the AD, use the poly felt padding that is used on the wing leading edges. This will give a really smooth appearance.
Before you do anything more with your “new” tank, pressure check it. Remove all jigs, pressurize it to not more than 3.5 psi (ref: FAR 23.965), brush a soap solution over all the joints and look for bubbles. If any leaks are found have them weld repaired. Don’t try to use anything else. It won’t work! Pressure check it again. Assure that the tank does not leak.
Install tank in center section. It is going to be tight but don’t skimp on the felt pads. Oh, an additional benefit: It will hold more than 46 gallons.
Tank jigs, Rubber mallet
Outlet plugs, 2-inch chip brush
Gas cap, Soap solution
Pressure gage (0-l5psi)