[CountryB]

I have 120V Mig welder. I've done enough welding of heavy steel (like 1/8" and 3/16") to be pretty good - when the pieces are the same thickness.

But now I have need to weld a heavy gauge "AN Bung fitting" to a light gauge fuel tank (in case your wondering - the tank is new so no risk of gas fumes).

How do I proceed welding this fitting.
Do I set MIG for very light gauge welding (like 20 Amp) and try to build up the welds around the fitting.
Or do set MIG for the "heavier gauge" of the bung fitting (like 40Amp) then direct the MIG heat/rod away from the tank metal (to avoid burn through)?
Or maybe some combination in between.

[CountryB]

From what I've been able to find on the internet, I should select the voltage and wire speed for the heaviest metal and then direct all the heat to the heavier metal. Some of the puddle will flow onto the lighter gauge.

I'm trying to weld a fitting (AN -8) to the bottom of a sheet metal fuel tank. The tricky part will be running the wire around the small circumference of the fitting without burning through the thin tank metal.

[MinnesotaDave]

Mig and fuel tank (liquid tight) welding are not a very good fit - but it's not impossible.

It's easier to weld a flat piece of thicker material to the tank first - then the fitting, in my opinion.

Regardless, this weld won't be super easy with a mig. Each weld start is a chance for a leak.

Clearly, after finishing test with water and not fuel.

Soap solution and very low psi air works too.

[CountryB]

That's a good idea.
After the fitting is welded to the tank, I will put in some Eastwood gas tank sealer, let it cover the inside around the fitting/weld real good. That should take care of any possible pin holes.