how to modify a gas tank without blowing up?

[79-GS550-L]

I am thinking of moving the mounting location for my petcock by about 1/2" to 1"... I think the PO moved it before and now I can't mount my new petcock without interference from the frame. I was going to bend the frame a little but it doesn't look like that will work well. I could easily move the holes with a dremel, file, drill, and some epoxy, but dremel + gas vapors = bad. I've heard of people filling the tank completely with water before working on it. After it's full can you dump some water out before cutting? And when you're all done, how do you dry it out completely?

How about if I just use hand tools, like a rasp and one drills that when you push down it twists (sorry I don't know the real name)? Can I do that without filling with water or is there still a possibility of sparks?

Also I wonder about epoxy being gas-resistant. I was going to use West Systems with some low density filler added, makes it about body-filler consistency but very strong, and easily sandable. Is there anything better I should use?

[rapidray]

I filled the last one I did with water & rinsed it MANY times but it rusted. I would PURGE the tank with CO2, Nitrogen, or Argon if I were you. That way you will eliminate the fuel vapors without the rust problem afterwards. You need to duct tape a supply hose to your tank opening to fill it with the purge gas & let it force the gas vapors out slowly. Take off the sending unit & petcock one at a time after you have started purging for about 10 minutes. That way you will push all the fuel vapors out. I would tend to want to use CO2 because of the way it displaces O2 so well. You will want to keep purging at a small rate when you start cutting or welding just in case to eliminate the oxygen & it's ability to fuel an explosion. Don't use any epoxy or anything other than weld to move the petcock or you WILL have leaks sooner or later! Good luck & PLEASE be careful! Ray.

[earlfor]

I would braze the tank using brass rather than trying to weld something that thin.

Earl

[Nessism]

I would braze the tank using brass rather than trying to weld something that thin.

Earl

TIG/MIG gets hotter but the total amount of heat added to the metal is lower than brazing.

[earlfor]

Not trying to be difficult or anything, but I believe either method, if gas vapor is present, will produce sufficient heat to make the task the "highlight" of your day. :-) I suggested brazing primarily because those old tanks can be too thin to even tig on.
Kinda like trying to lay a bead on a tin can end. :-)

Earl

TIG/MIG gets hotter but the total amount of heat added to the metal is lower than brazing.

[SqDancerLynn1]

The real problem is that you have the wrong petcock ??? L models require special low profile petcock

[txironhead]

Save yourself a lot of grief and potentially a ruined part (of your bike, if not your body), and verify that it's the right petcock. Does it look like the mounting point has been moved previously? If it has, your best bet is to just find another tank.

[drhach]

I assum since you are welding it that you don't have concerns about the paint. If that is true, take it to a radiator shop and have it boiled out. After that, ther is no concern of fuel vapors being present. Also, after it is boiled out and welded, you can bring it back to them and have it coated to prevent rust. I have an old tank that I tried doing some MIG welding on. It was tough. Also, I'm not that great of a welder, so that probably didn't help. That metal is very thin and it gets thinner at the bottom of the tank because there is usually some rust.

[79-GS550-L]

Ouch, this is starting to sound more difficult than I realized.

The real problem is that you have the wrong petcock ??? L models require special low profile petcock

NOW I think I have a low-profile petcock. The previous petcock was an obvious aftermarket one, it was not vacuum operated. And the mounting location does appear to have been moved. With this new (appropriate) petcock, I can squeeze the tank back onto the bike, but the vacuum nipple is pointed straight into a frame bar, there's about 1/32" clearance to attach my vacuum line.

I do have a MIG available to work with. Will that be too hot? If it is, what happens, does it deform the metal? The metal seems fairly thick at the mounting location, much thicker than a tin can at least There's really nothing other than weld that will hold in gas without dissolving?

Finally, the inside of the tank has been coated with some sort of white stuff, I guess to prevent rust. PO probably had it coated after moving the mounting holes the last time I'm not 100% sure of it's condition, it might be intact or it might be peeling inside. If it's in good shape, would that allow me to fill with water? I don't have a source of argon or co2 etc to purge with. Can I dry it out with a heat gun afterwards?

If I do weld with water in there, I can't get water filled to the brim, since there are holes there is it safe if I rinse the whole thing out very well and there is an inch or two of air space at the top? I assume I can't really weld underwater.

[txironhead]

The white liner is probably Kreem. It's not the best thing to have in there, as it tends to break down over time and clogs everything up. POR-15 makes a tank liner kit that is far superior.

I honestly think you'd have better luck trying to find a useable replacement tank. You can look in the Parts for Sale section here, post in the Parts Wanted section, or check out Ebay or local salvage yards. You've already got the new petcock, you'd just need to line and possibly paint the new one.

The issue is not warping, too hot a weld will burn through thin metal, and then it's a major hassle trying to get the tank to seal properly.

Why did you decide to not use the aftermarket petcock? If you decide to absolutely not use it, I might be interested in it.

If you do fill the tank with water and attempt to weld it, the best thing to do to prevent rust would be to order the POR-15 tank kit. Fill it with water, do the cutting/welding, drain and use the kit. The kit also comes with repair patches, etcher, and a solvent that is good for removing any previous liners such as the Kreem liner I suspect you have. The external repair epoxy is gas proof and would be excellent for making sure the weld is leak-free. You can read the instruction manual and more info on the POR-15 website. Great stuff, probably last forever.

[79-GS550-L]

According to the West System website, their epoxy is safe for use in building gas tanks :-D

http://www.westsystem.com/ewmag/18/tanks.html

They give tips for making sure it's gasoline safe (mix slightly more resin, slightly less hardener; double-mix it; heat cure; then scrub with water). The article is boat-specific, but I can't see any reason why it wouldn't also work for a motorcycle tank. Gas is gas, right?

So, if all I need is this epoxy (which I have a ton of) and some hand tools, do I need to fill the tank with water? Is there any danger of a rasp causing a spark? I might actually be able to get some CO2 to use instead of water.

[79-GS550-L]

Hmm, should have read a little more carefully... looks like there are problems with the new ethanol additives dissolving the epoxy.

I'm going to have to go take another look at the tank and see what else I can come up with.

[duaneage]

but the vacuum nipple is pointed straight into a frame bar, there's about 1/32" clearance to attach my vacuum line.

Can you put a 90 degree T on the vacuum tap?

[t3rmin]

OWith this new (appropriate) petcock, I can squeeze the tank back onto the bike, but the vacuum nipple is pointed straight into a frame bar, there's about 1/32" clearance to attach my vacuum line.

Can't you just pull off the backside of the petcock (the four screws) and rotate so the vacuum nipple is pointing differently? I don't think there's any reason it can't be oriented in any of the four possible positions...

[79-GS550-L]

Can't you just pull off the backside of the petcock (the four screws) and rotate so the vacuum nipple is pointing differently? I don't think there's any reason it can't be oriented in any of the four possible positions...

That would be the best by far... I think I looked at that and it wasn't possible, but I don't have it here with me so I'll have to check later.

but the vacuum nipple is pointed straight into a frame bar, there's about 1/32" clearance to attach my vacuum line.

Can you put a 90 degree T on the vacuum tap?

Not enough room for a T... maybe if I chop a bit of the nipple off. Also I suppose I could try to cut a section out of the nipple and bend it 90-degrees with some JB weld or something. I was hesitant to tear apart a $50 petcock, but since it is looking harder and harder to modify the fuel tank, maybe that's just what I'll have to do.

I also heard that spray-on truck bed liners are usually HDPE which is apparently safe for fuel tanks even with ethanol. But that didn't come from a reliable source so take it with a grain of salt!