Rusted Gas Tank Repair

My wife owns (and dearly loves) an '83 Yamaha Maxim 650, and it's in pretty decent shape for a bike that is nearly a QUARTER CENTURY OLD, which ain't sayin' much. There are inevitable problems, among them, a rusted gas tank. Normally, rust only forms on the left side, which is the kick-stand side. The P.O. (Previous Owner) was apparently quite diligent about storing it on the centerstand, as BOTH sides of the tank were rusted almost equally.

On the right side, you can actually see a "water line" which is perfectly horizontal when the tank is on the bike. There is a scientific reason that the rust forms a line at the top of the water, which sadly, I do not know. My long-held theory was that rust, a.k.a. iron oxide, requires oxygen to occur. Unfortunately, that theory has proven to be -- wait for it -- full of holes because in the case of a gas tank, there is gas floating on top of the water, which should exclude air.

Zina's bike has damage on all four "sides" -- the outside, which is the cosmetic side, and the tunnel, which is a non-cosmetic side. I find the holes by using a rotary wire brush. This also has the benefit of stripping paint & rust pretty agressively. It's pretty amazing how many holes show up when you get busy with the wire brush. Holes are like cockroaches -- if you see one, there are a bunch more that you can't see. Ick. The P.O. had patched this tank with JB Weld, and unbelievably, it was mostly holding gas. Presumably, with the paint.



Next, the shape of the patches are sketched on the tank with a Sharpie. The outside was a relatively simple compound which, with my modest level of metalshaping skill, I can achieve in one piece. The tunnel contains at least two different compound curves. I opted to do this section in two pieces.





To determine the flat shape of the patch, I use paper, which is "smooshed" to conform to the surface I'm repairing. Notice that there are tucks in the paper, which are circled. The tucks indicate where the metal needs to shrink. The paper is cut out and outlined on the sheet metal. The parts are cut out of mild steel. I used 12 ga. because I have a large amount on hand. 14 or 16 ga. would be easier to shape. I normally cut the patch about 1/8" larger on all sides, which is trimmed off as the patch is shaped.



Notice that I do the shaping before I cut the rusted part out of the tank. Ideally, the patch will perfectly conform to the tank before any cutting happens. Begin the shaping by FOLDING the patch until it closely conforms to the surface. The better you make it fit now, the easier the shaping will go. If you recall, I mentioned the "tuck" in the paper pattern -- when the sheet metal is bent at this stage, there will be a sharp "sticky-outy" where the tuck needs to happen.

There are only a limited number of things you can do with sheet metal: cut it, fold (bend) it, stretch it, and shrink it. I use a shop-made stake held in a vise to shrink the shape into the patch. The stake has round ends, and I hammer the sheet metal just in front of it's contact point with the stake. This forces the metal to wedge in on itself, and it actually becomes thicker. There are two kinds of hammering -- hammering "ON" and hammering "OFF". If you hammer on the dolly or stake, it rings. If you hammer off the dolly or stake, it makes a dull "thock" sound. This is what I want to hear when I'm shrinking.



The next thing to happen is to cut the rusted metal from the tank. I use an pneumatic abrasive cut-off wheel, but you could just as easily use a Dremel tool, or drill a hole in the waste part and use a saw. I wouldn't recommend an electric jig saw, as you'll probably poke holes through the back of whatever you're cutting, and it just might be your leg that gets poked. Speaking of holding the work, I find that an old tire on the workbench protects the tank, and keeps it from rolling off onto the floor.

You'll want to cut the hole slightly undersize. If you get too crazy cutting the hole, your nice new patch panel will be too small. That would just plain suck, so don't do it. You can put the patch onto the tank and draw a "DO NOT CROSS" line, but ideally, you want to be on or slightly inside your original line. Once you have the hole cut, use a file, Dremel or die grinder to straighten up the cuts. There is no point in making extra work for yourself by having to follow wavy lines.



The fit of the patch needs to tight. The rule of thumb is that the gap should be no wider than the thickness of the material being welded, or no wider than the welding rod you're using. In a perfect world, the gap would be about 2 sheets of paper, and perfectly uniform all around the patch.

At this point, the patch should still be oversize. Use a coarse file, a belt sander, or an abrasive wheel to tune the fit. It's good to have a reference point that allows you to replace the patch in exactly the same place each time. It can be Sharpie marks, scratches, punch marks, or even a tack-welded tab.

The tab is a nice feature for another reason -- it's really easy to drop the patch into the inside of the tank.

Next step is tack welding. Sometimes, I use a magnet to hold the piece in alignment while I put a very light tack near my reference point. Once there is one tack in place, I can remove the magnet and reposition the patch as needed.

A weld shrinks as it cools. This shrinkage can cause distortion if it's not acounted for. Your most common line of defense against distortion is to tack early, tack often, tack tidy. I say to tack tidy because it's a real drag to have to grind out a big fat booger of filler rod if you need to reposition the patch. Also, due to the tack welds shrinking, it's wise to leave a very slight gap when you make your first couple of tacks.

Flat work requires more tacks than highly shaped work because the work with shape is quite stiff already, and can resist the weld distortion but flat sheets can't. Regardless, I try to tack about every inch, or where there is a contour change.

Even with all the tacking preparation, sometimes the patch will slip under the tank, or vice-versa. I start with sharp objects such as a chisel or a box knife to persuade the two pieces to return to their rightful place. This doesn't always work. Drill a small hole at the joint and use a screwdriver blade to try to force the edges apart. Sometimes they will spring back into alignment once the overlap is relieved. Sometimes, you just need to pry them. Don't be afraid to grind out some tack welds if you are dedicated to making the job as perfect as possible.



OK, here's where it gets interesting. Once you have the patch entirely welded up, and you carfully inspect the results, you realize "Crap... I have a bulge the size of a dime." One of my favorite tools would have to be the "Gas Axe" or the "Big Blue Wrench". Of course, I'm talking about the Oxy-Acetylene torch. They deliver a lot of heat, NOW.

Use a carburizing flame -- i.e. slightly rich, which is cooler than a lean flame. I like to bring the bulge up to a dull cherry red before starting to hit it. Care must be taken -- a moments inattention will put a hole in your patch.

I use "Hit" in the loosest of terms. Because the steel is very malleable when heated, a tiny tap will do the job. Also, choose repitition over force while the metal is hot. Lots of little taps are much more controllable than fewer, harder hits. Also, you can't just hit it from one direction, the face of the hammer needs to "orbit" around the surface in order to bring it down to the desired shape.



As you progress, check your work with a file. Skim a file across the surface, and it will mark the high spots, and adjust as necessary, but don't go too far. Also, use the file to cut down the weld bead to be level with the surface of the tank. Don't attempt to level the work with a file -- it will remove too much metal.



Now, using a random-orbit sander with pretty coarse pad, smooth the work. This is how rough you want it in order for body filler to stick well. The dark areas will be filled with Bondo. At this stage, I also use a small-diameter abrasive wheel in a die grinder to dress the welds along the seam.



Now, comes the bottom of the tank. Since this side is not a cosmetic face, I don't spend a lot of time doing beautiful work on it. This means that the rusted tank is not cut out, and the patches are applied over the tank. The patches need to be large enough to get to non-rusty metal because you can't weld to thin, rusted metal.

My major time saver is to use the gas axe combined with a pneumatic needle scaler. The scaler is basically 25 little tiny hammers that go tap tap tap on the soft metal hundreds of times per minute. This is not a subtle technique. The resulting finish is ugly as a mud fence, but it literally only takes two minutes to form a complex patch.

In retrospect, I should have had more confidence and attempted these as a single patch. After all, until the patch is formed, it's only held on with a couple of tack welds. Nothing ventured, nothing gained!

First, the patch must be patterned as usual, and then tack welded to some good metal before it's heated. If the patch is squiriming around while it's being hit, a second tack is useful.



The results are not particularly good looking, but I accomplished both sides using six patches in less time than required to do either one of the outside patches. What I find amazing about this technique is that the thickness of the doubler has "printed" through the patch. It will force the patch to conform very tightly to virtually any contour.

Also worth noting is that on this particular tank, the mounting tab doubler was very close to some rusted areas. It didn't occur to me until after I had welded the patch to the doubler that it was just spot welded, and would leak. I had to cut the patch and the doubler back quite a bit in order to be able to weld to the tank skin.

The final three steps in this repair are to do the "Soap Test", the "8-Hour Test", and treat the interior of the tank with Kreem or other tank sealant. Both Yamaha and Permatex also sell tank sealants.

The "Soap Test" just means that you introduce a ***very small*** amount of compressed air into the tank, and paint the outside with soapy water to look for leaks, which, of course make bubbles. DO NOT over-do it with the air. A friend and mentor tells a tale of trying to "blow out" the dents in an aluminum TT500 gas tank using a torch and compressed air. He had his compressed air regulated to 25psi, but that was still too much -- the tunnel blew outward, creasing the tank across the filler neck. The hollow object that *used to be* a gas tank hung on his shop wall for years as a conversation starter.

An excellent safety measure is to hose-clamp a section of bicycle inner tube between your blow-gun nozzle and the gas tank. This will very effectively regulate your pressure to safe amounts.

The "8-Hour Test" just means the tank is filled with water, and set on a "telltale" such as paper towels, and monitored for leaks. I devised this test after blindly trusting that my welds were sound, which as you might have guessed, was the wrong thing to do.

5 gallons of etchant (i.e. acid) leaked out of a tank and the fumes rusted every iron tool in my shop. It was a painful, memorable lesson. A sealant is absolutely manditory because welded metal is super-susceptible to rust, which left untreated, will quickly put you back to square 1. My only advice about tank sealant is follow the instructions TO THE LETTER. Most unpleasant experiences with Kreem are due to user errors.