cut down gas tank.Help

[racer x]

      OK I bit off more than I can chew. I do that from time to time .
  This time I am trying to make a new gas tank for a motorcycle.
I took a stock tank and cut the top off. Easy as pie. Then I bought another tank with a flat top section and cut it's top off. Are you with me so far. Then I used my very limited metal working skill and made the top fit like a lid to a pot . As tight as I could . Bending and hammering for a week .I left about 1/2 inch of overlap and then using even less welding skill I welded it together. My plan was to braze the finished seam and then check it for pinholes .

You guessed it .there are a lot of pin holes. I am going to go back and smooth it all down and clean everything with a with brush . to get any impurities out and trying to keep it horizontal . I am going to braze some more.
Should I keep brazing till I get it sealed or just go with a liquid tank liner or both

   the seam is only along the top . And there will not be any pressure inside but I don't want it to leak of course.
 

[doug odom]

How did you weld it?  Tig.,Mig or O/A gas?

[racer x]

I used a wire fed MIG welder . I made two passes . first spot welding it to hold everything together than I went around the edge seam trying to fill in between the welds.
 Then I used an acetylene torch to braze the edge. It looked good but when I pressurized the tank the bubbles seeped out along the side.   

[Peter Jack]

Information for next time. Grind all the tacks to as thin as you dare and then weld right through them. If and when you do stop, grind the end of the weld to nothing, then start on the weld and weld right over the end that you just ground. Every time you start on an unground tack or the unground end of a weld you have a cold start and likely porosity. The even better way to weld the tank is to have someone tig weld it.

As for now, all you can do is braze it as once you've hit it with brass there's no going back.

Either way cleanliness is the most important item. Make sure you're not dealing with any paint, oil, etc. Mig especially doesn't like paint.

Good luck!

Pete

[pookie]

Hi Racer X,  A couple of suggestions If I may. Like Peter Jack said, grinding the welds down to smooth out is a good idea, but try not to grind on the parent metal of the tank. You said you were going to wire brush the welds to clean them up, I suggest you use a small stainless brush and if you are using that brake cleaner I see in the pics you posted, DON"T USE IT!!! Get some acetone, clean with ss wire brush and wipe weld area down a second or third time with a clean rag and acetone to remove any vestages of dirt or residue.You didn't say what size tip or brazing rod you were using, or what kind of flux you are using. The torch tip size and the correct rod size can be determined by a good welding supply shop,, the flux can get a little tricky, too much, not enough, but it maybe best  in your case to use pre-coated brazing rod. The correct size flame is also a factor, again hard to describe here, but that good welding supply shop should have pictures which will help you to get the flame right. When starting to braze, I would turn the tank so the top was on the table side, so I could run my puddle into the seam of the thicker part of the tank. But I would be heating the metal from thicker side to to the thinner portion so I could have better control of the brazing material into the seam. Remember, you can maintain control of the heat by moving your torch away then back into the puddle, that way you won't be blowing holes or warping the parent material. Also, dip your brazing rod into the puddle take the rod out only so far so you'll have a preheated rod for the next dip. Don't melt the rod with the torch flame. have the puddle do it. So there you go, not much to brazing..... Yeah Right.. Oh 1 more thing when you are positioning the tank make sure you are comfortable, even to the extent of building supports for your arms so you can maintain a comfortable position at all times... Hope this helped, good luck Mike R.

[racer x]

Thanks guys. I sure do appreciate the help. I am going to go for it again . I think if i am careful and I do have a NEW ss wire brush and I don't use brake kleen .That is for the SAAB engine I am building at work. I should have tried a practice tank first but did not think about it enough.

[manta22]

Racer-X;

I'd recommend using Harris #17FC nickel silver braze welding rod together with Harris #17 flux. This isn't the same as the usual brass brazing rod-- it is far stronger and you can run a bead if you wish; with a bit more heat it thin- flowing but not as much as brass rod. An alternative is Welco #11FC but it isn't as thin- flowing at high heat, it tends to fume instead. It may be easier to find.

As Mike & Pete have already said, make sure the work is very clean, both inside and out. Paint on the flux with a small acid brush and set your torch to a medium neutral flame. Even though the 17FC rod is flux- coated, I paint the joint with extra flux to make sure the joint is protected before the rod flux melts and flows into the joint. A common mistake is to use a far bigger torch than necessary when brazing thin sheet metal. No need for something large-- small is better. It only takes a dull red heat to braze weld-- don't overheat the joint.

Regards, Neil  Tucson, AZ

Regards, Neil

[racer x]

Today I made another braze attempt. It went MUCH better with all the help. I made a bigger wider bronze area over the seam . I got a dark red area going and let the metal melt the rod . Before I melted the rod with the torch. Then I pushed the puddle along keeping it smooth and letting it freeze slowly behind. it looks good , But there are a couple tiny pinholes. Nothing like yesterday

I did not think this would be easy .  I am embarrassed at the lack of preparation I used from the beginning. if this fails i can only blame myself.

The first thing is my equipment . I am using an antique torch that an old friend gave me . He used it for 40 years soldering radiators . It dose work well and has a small tip but probably to big.

The second thing is my skill level . Had I thought this through better or even done  a cursory study .That I have done since starting . I should have given the parts to a real welder .
That is not my natural inclination . I try to learn something . Problem is I only learn from mistakes . Mine and others. I cant afford this mistake .

Then there is my environment .I don't own a welder. I am using a giant steel workers MIG thing I call old sparky . I am working on a greasy work bench and trying to get it all done at lunch time.

O yes hindsight is 20/20. 

Any how in spite of all of this I managed to braze the whole thing with only a couple tiny leaks .The NEW problem is the top is starting to warp from the heat. It is shrinking and curling around the seam.
So at this point I have to stop . I cant heat it any more or it will tear apart and if I grind any more I am afraid it will thin out the metal sides too much .

From this point I am going to use a chemical tank liner called Kreem.A three part acid etch and polymer coating that works very well in old rusty gas tanks. And the painter said he will fill and smooth the outside with fiberglass filler to help even more.

I think (hope) it will be OK . The fuel level is well below the seam . I wont leave fuel in it when not at the track . So the worst that can happen is a couple paint blisters. I have learned a lot in the last couple days and am now motivated to learn to weld . So all is good . Thank you  very much for the help . 

[manta22]

Racer X;

If you are having warpage problems it is due to too much heat. Brazing is done at a temperature that keeps distortion to a minimum.

I'd suggest getting a small torch to use for brazing & welding sheet metal and thin- wall tubing. Big torches are cumbersome and can't be turned down well enough for light guage stuff. Get some scrap steel (16 ga or thinner) and practice, practice, practice-- there are no shortcuts.

Regards, Neil  Tucson, AZ

[Freud]

It' not the size of the torch.....it's the size of the tip.

FREUD

[Dr Goggles]

It' not the size of the torch.....it's the size of the tip.

FREUD

many would say it's neither.....it's the way you use it

[racer x]

this is my tank so far . I will seal it this weekend and the painter will hide my sins.

I have begun to study the basics of heat control and welding.I have a lot to learn.

[racer x]

the tank on the bike

[racer x]

before the new tank I had to stand in the foot pegs to get my back flat.

[racer x]

After shot is sitting down and I can move my chin all the way to my chest .