Unfortunately, plastics are not easy to form. Once they have a few years behind them, ozone, chemicals and ultraviolet radiation cause them to become brittle. I’ve tried doing plastic welding and forming bent plastics back into place on older vehicles, and it never really works well. Heat and chemicals in the fuel (ethanol) are the likely culprits. It way have warped or expanded when the vehicle was operating.
Once you took off the bolts holding the fuel tank in place, it probably relaxed to a natural position and now doesn’t fit right. The bolt holes probably don’t align, either.
You could make a large wooden clamp that may reform the fuel tank over time, but I’d be concerned of invisible micro-fractures and the integrity of the fuel tank.
I recently removed the fuel tank from my Triumph Sprint ST to replace the throttle cable. Since I had to wait a few weeks for the new cable, the fuel tank sat off to the side. After the cable was replaced, I went to re-install the fuel tank only to find it would not drop in place between the frame rails or fit front to rear.
After searching online, I discovered this is a common problem with many plastic fuel tanks and several manufacturers, but I found no solution. Is there anything I can do to shrink this fuel tank back to stock dimensions?
The cost of a new fuel tank is just so expensive….