I wonder if the friction of a metal bit drilling a hole in a tank might not send a spark into the fuel.
I almost wrote that soldering shouldn’t be too dangerous. Then I looked it up to save me from being responsible for someone trying it.
Your soldering head temp should be between 600F and 700F.
Gasoline auto-combusts between 538F and 853F (depending on octane)
You are definitely taking your life into your own hands.
Friction wouldn’t be much of an issue, probably no sparks there because of the tank metal alloy, and no heat because the liquid fuel inside would cool it. But if drilling a hole in a metal tank you would have to use a drill. And unless you are using a hand brace and bit it would have to be electric. Electric Drill Motor Brushes arcing + Gasoline + Oxygen = bad news...
Gasoline has a fairly narrow flammable range. It only burns when the mixture is between 1.4 and 7.6 percent gasoline to air... That is why a car that will not start is often said to be “flooded”. The inside of a fuel tank that still has gasoline in it will typically be too “rich” to burn unless some fresh air has been inadvertently been added to the tank by changing temperatures etc... But they typically do not blow up like in the movies without some help from an incendiary device.
It is the fumes from gasoline that cause the most danger. People are often careless when using gas to light a brush pile on fire and end up catching the can on fire which if allowed to melt can put on a pretty good show when the gas starts spilling out.