So I have to modify the stock steel gas tank in my 2003 Chevy 3500 boxvan. I need to section out about 8" off the rear of the tank.
I have the tank out, and empty of gas. I currently have a 12v fan pushing air in the old fuel pump/sender unit hole to dry it out.
how can I make sure ALL the vapors are gone? and it will be safe to cut into (lots of sparks)
is rinsing it out with water going to do any good? a different solvent maybe?
input please
Insert all sorts of hazard warnings and disclaimers here...
OK, with that covered, I have not done this, but what I have read is to fill the tank with an inert gas to displace the oxygen. The idea is to keep any lingering fuel vapors from igniting while welding. I've heard of using welding shield gas to engine exhaust pumped into the tank while welding.
Please don't blow yourself up.
I did this to add an internal swirl pot - I pulled the tank, flushed it with water and then left it out to try for an hour. Then I cut the top off and threw a lit rag in it the hole.
When it didn't explode I assumed it was safe and proceeded. YMMV 
I've never done it, and don't plan on trying. What I've heard is to fill it up with water until about 1" from the weld site, THEN back purge it with inert gas.
Yeah, I've heard the welding gas or water trick. Sadly, my balls aren't big enough. I'd take it to a shop that specialized in repairing fuel tanks. That way, if the tank blow us my wife won't be rich without me.
I need to cut the tank open and then weld. so filling gas or water while I work is not an option.
keep'em coming
jhaas wrote: I need to cut the tank open and then weld. so filling gas or water while I work is not an option. keep'em coming
I don't understand the problem then... the tank is empty. Unhook the fuel lines and fill it with water until it overflows back out the fill neck. Drain it. Cut it. If you cut it while it's full, it will drain faster. Weld it back up. Fill with gas and two bottles of dry gas.
Set it out in the middle of a field. Get a really, really long stick. 25 feet oughtta do it. Set the end of the stick on fire, and light the tank. Film and place on youtube.
GPS^^ I had a gas tank explode on me last time I welded one. It was dry for days before I tried. I blew it out with a shop vac for an hour, dropped a lit match in it. nothing. but as soon as I struck an arc...BOOM
If you aren't going to leave water in it while you weld, at least fill it up with water and drain it, let it air out for a month+ if you can. Gas will leave all kinds of explosive/flammable residue. An empty fuel tank with vapor will be much more likely to explode than one full of gas (note explode and not catch fire).
I heard this somewhere, so don't blame me, etc.
Got an oven big enough for the tank to fit? Bake it for an hour or so at 200F, should flash off everything. Then finish at 350F for another hour.
No oven?
Then build a pit fire, string the tank over it with wire, and bake it that way. Use a BBQ thermometer to track the temp (use binoculars to read it), raising or lowering the tank as required.
Let it cool, and weld away!
jhaas wrote: GPS^^ I had a gas tank explode on me last time I welded one. It was dry for days before I tried. I blew it out with a shop vac for an hour, dropped a lit match in it. nothing. but as soon as I struck an arc...BOOM
Yeah, the guys I've talked to about welding on tanks say that the problem is porosity. Steel is porous enough that, with use, it will absorb a fair amount of fuel that you won't see when you check to see if the tank is dry, won't show up if you drop a match in (since its in the metal), but once you heat the metal sufficiently...
The oven idea might work... cook the embedded fuel out of the steel at a low enough temperature that the fuel doesn't ignite...
I have modified a few tanks over the years and never had an issue with just flushing and welding.
I'm glad I don't ask first or I might never get anything done 
I've done this a few times. I filled the tank with water and kept a hose running in it while I cut it. Then I did whatever work needed to be done inside (baffling) and welded it back up. There was no smell of fuel at all after it had been cut open.
After blowing it out thoroughly, you could WHILE OUTDOORS seal it up and pull a vacuum on it with a shop vac OUTDOORS make sure you dont pull enough to collapse it, if so, just poke a hole in the duct tape sealing the sending unit hole to bleed some air in.
Giant Purple Snorklewacker wrote: I have modified a few tanks over the years and never had an issue with just flushing and welding. I'm glad I don't ask first or I might never get anything done
i have done the same thing.......
I have one to weld, adding an intank electric pump in the Vanette gas tank.
My plan is I will wash it out several times with hot soapy Tide. A few/many hours with the electric leaf blower and then drop in a pound or two of dry ice from the Airgas Co. or Fishmarket?
Wish me luck.
In reply to Gasoline:
Dry ice, thats a good idea.
In reply to Gasoline:
good luck you should be fine..........
BTW does anyone have dibbs on the spitfire 
Gasoline wrote: I have one to weld, adding an intank electric pump in the Vanette gas tank. My plan is I will wash it out several times with hot soapy Tide. A few/many hours with the electric leaf blower and then drop in a pound or two of dry ice from the Airgas Co. or Fishmarket? Wish me luck.
The dry ice is an interesting idea.
good ideas, thank you.
I don't have a month to let it air out. I need to get this done this week. I'll wash it out and go from there.
cutter67 wrote: In reply to Gasoline: good luck you should be fine.......... BTW does anyone have dibbs on the spitfire
A couple of weeks ago I was trying to give it away free....no takers. Ask again in a month or two.
It is still kinda cool. My first car was a Spitfire. Might just tuck it away for $2014? Turbo Ecotec? I'm thinking?
Just too small for a LS motor. 
'splain the dry ice for me?
And be wary of the shop vac idea. I seem to remember somebody did something similar and the spark from the motor ignited the fumes for a very interesting display of pyrotechnic force. He also lost most leg hair on one leg.
DrBoost wrote: 'splain the dry ice for me?
Dry ice = CO2 = inert shielding gas, without the hassle of figuring out how to hook it up to a cylinder
JoeyM wrote:DrBoost wrote: 'splain the dry ice for me?Dry ice = CO2 = inert shielding gas, without the hassle of figuring out how to hook it up to a cylinder
Man, this forum is smrt...I mean smart!
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