Replacing a Piper J-3 gas tank
[The following was posted to the Piper Cub Builders List by Len Buckel, in answer to the question: "How do I get the gas tank out to replace it?"]
Bill: Either repairing your tank or replacing it will take the same route. The tank will have to come out through the bottom as you have would have to remove everything forward including the boot cowl to get it out through the top.
Remove the two tubes from below the tank after draining the tank, disconnecting the fuel line and the fuel shut off control.
These tubes may be hard to get out if they have been in place a long time or if they were not lubricated before being installed. There will be a 1/4" bolt at both the top and bottom of the tubes. Remove the bolts and see of you can turn the tubes. You will probably have to use channel lock pliers or some other method of turning the tubes to loosen them. If you have to use some type of pliers, grip the tube at least four inches away from the point where it goes into the outer tube. Your are going to have to work the tube in one direction or the other until the far end will come out of its outer tube. As soon as the far end is out, work the other end out.
After both tubes have been removed, you will need to loosen the tank straps. The bottom tank straps should be installed on top of the tube that the strap bolt go through. The strap bolts should have been installed in the top straps so that they do not move when you take the nuts off. After the nuts are off of the strap bolts, you should be able to push the bolts up through the tubes that they mount in until the lower straps can be moved sideways off the tubes and removed. After the lower straps are removed, you will be able to work the tank down and out. The straps are different so mark them so you will get them back in the same location when you replace them.
The flange for the fittings on most of the fuel tanks are soldered in. Yours may just need to be resoldered.
Make sure that it is not your fuel shut off valve leaking before going to all of this work. This valve can and often leaks especially if you turn it off and on all the time.