Originally Posted by CroMagnum
So what happens when part of the pipe you don't want to change balloons out and you've lost your dimensions? Not to mention the thousands of PSI you'll need to actually bend metal. Sorry, but while this alleged forming technique may work in Internet Fantasy Land, it worthless, expensive, and dangerous in the real world.
Whoa whoa whoa hold the phone. Why are we talking about expansion chambers? The original topic was forming gas tanks, which probably can be done.
EDIT: Just looked on wikipedia:
How expansion chambers are made
Two flat representations of the required finished pipe are cut out of sheet metal. The edges of the two identical flat cutouts are welded together forming a sandwich. On one end of the pipe a fitting is welded and high-pressure water is pumped into the cavity between the sheets. The pressure inflates the flat sheet into its final rounded shape. This method can be quicker than hand forming and only slightly more costly in tooling, however it requires a number of trials before a finished design as accurate as hand formed or stamped can be produced. All curves must be made in a single plane so cutting apart and re-welding is often required but the final product can be as good as a stamped pipe if enough care is taken to be precise.