Well the OP's question was about the safety of welding on a tank that has had fuel in it.

What should I expect to leak? After fixing a couple pin holes it held pressure fine, and 9 months later doesn't leak. I'm not saying the quality of my tank is good for a long term mod, all I'm saying is I welded on a gas tank and didn't die.

You missed the part where I stated that If you can get or have access to welder.... . Probably have access to argon and jamming a hose in the tank to purge isn't hard. Leading me to say why not. I'm glad that you were able to weld it and not hurt yourself but when there is a easier way why not do it

Because we're "hot rodders" and "Do-It-Yourself" people...


I can afford to buy anything I like... but I also have a lot of tools and a very nice shop... so I can do things myself or for others. Sometimes it has nothing to do with buying. The saying "built not bought" comes to mind.

You apologized, so I'm not going to get into any heated debate here. But I want to discuss this so no-one tries anything dangerous here. I'll assume you are talking about head pressure. 12" of fuel is about as deep as you'll get in any fuel tank in a car. 12" is about .5psi of head pressure at the bottom of the fuel tank and it gets lower as you go up. Pressure testing to .5 psi is fine, but that's not really considered a pressure vessel.

Factory tanks are resistance welded around the edge sometimes with some seam sealer in there to help seal it. It's not the best weld. Just bending the weld joint is sometimes enough to make it leak.

Let's assume you have a 3' x 2' tank (36" x 24") That's 864 in^2. If you pressurize that tank to 5 psi that's 4300lbf trying to rip that weld apart. If it were a round tank like a tanker it would withstand that pretty easily. But, since it has corners, those are the weak points and as it deforms puts some really bad stress on those welds. It's not safe to pressure test that high because it was never meant for that.

Just filling it with water and letting it sit is usually good enough to find a leak.

What I did was mostly an experiment and my first time welding a gas tank. I thought about purging it with argon, but since I can't see argon and fuel vapor, having water I can actually see and know for certain how much vapor had been displaced, I did it that way. I do need to set up my tig to be able to back purge for welding stainless in the future, but with my current regulator set ups, it was easy enough to grab the water hose.

Again, I'm not saying anything I did was the best way. I was just sharing to show that welding on a used tank isn't the explosion hazard a lot of people think it is. :)

This seems like a really (or at least potentially) expensive and stressful way to save money/be creative. I get the thrust of the venture, but over the summer, I found out how ridiculously difficult it is just to get all of the gas out of my Falcon's former gas tank. I gave up and threw it in the truck headed for the dump . . .

Stay safe!:peepwall:

just sayin, you brought it up not me. :stirthepot:

no dog in this fight if you can't see that. :lmao:

you are completely right lets stop the heated debate and insults (myself) and yes discuss this the right way, please see the next post

ok let me state here what i know and dug up ,
# 1 you cant see argon nor fuel vapour but you might not be aware that
argon is a inert gas that aids in welding as IS heavier then air which
is why alot of people will use it to purge tanks of flammable vapours
a fuel tank can be purged in just a few short minutes
#2 setting up your current TIG welder for back purging is quite simple of
course it depends on what you are welding but for the most part its as
simple as turning the tank on and set the regulator at barely open to flow
a few psi down a hose into the part being welded a little tape to hold it in
place and away you go . also tape off all openings except one
#3 i went out of my way to speak with my ex's step father he owns a local
radiator,AC,etc shop. he has welded thousands of tanks over the years
but whenever possible avoids it due to cost . his process is use a hotsy
hot water pressure washer getting the tanks to well over 100 degrees
immediately blows them dry with compressed air then lets them sit for a
day followed with whatever welding and then a pressure test in a water
tank. the way he pressure tests them is sets a regulator to approx 20lbs
then cracks a ball valve attached to it for 2-3 seconds .this will fill the
tanks to 1-3 psi after which he dunks them into his water tank but a soap
water mix will certainly work. any higher pressure will balloon the tanks
rendering them useless . i stated i had welded transport tanks earlier and
we pressure tested them at a higher pressure but this was due to the
much thicker material and construction (cylindrical) .
#4 PLEASE understand im not trying to be a fabrication snob i just couldnt
stand knowing that someone got hurt when i had the knowledge to help
guide them along safely and correctly
#5 Jim told me that the garage beside his rad shop was run by some italian
fella who used the water technique . his way after washing out a tank
dry it with a rag lit on fire on the end of a old fishing pole (needless to
say there was a few tanks that went up with a hell of a kaboom) he
then graduated to the water technique the problem with this is any
remaining vapours now get concentrated to the remaining airspace in the
tank with can increase the chances with a kaboom .

hopefully this can and will help anyone and everyone who is considering doing something such as this at home . FOR THE RECORD I WILL NOT TAKE ANY
RESPONSIBILITY FOR ANYTHING IF YOU BLOW YOURSELF OR YOUR HOUSE UP
when i asked jim what the best home made process is he said anything is doable the biggest thing is getting the tank clean he suggested a shop such as his,have the tank steam cleaned by someone else then weld it yourself, and last was the one i suggested clean it ,dry it ,air it out,argon purge , he did not suggest the water technique for the reasons i stated quite a bit earlier. i would also like to say that yes i chirpped about the appearance of the welds , i will say that im well aware some welds dont have to be good looking to be strong and vice versa . i have done quite a few over the years which landed on both sides of the fence it just goes with the territory of being a custom steel fabricator . what a poor attempt of my chirp was trying to state was it didnt look like the welder was not set up correctly for that thin of material and it looked like a bunch of tacks . a bunch of tacks can be strong when done right but can also cause weld faults in the form of inclusions and crater cracks. please if anyone has any constructive things to add or comment on chime away.
thanks guys Ryan

With as cheap as tanks are for the more popular models, I agree it's not worth the hassle if you have one of those cars. Especially now that there are EFI tanks as cheap as they are for some cars.

Good info, thanks.

The idea with what I welded was to just make a decent stitch weld around the ring. I had a hard time not burning through the thinner metal and had to weld up some holes. You mentioned galvanized can cause some popping so I wonder if that contributed as much as the metal being thin did.