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Tech Tips/ DIY

Trimming The Fat: Chopping A Custom Tank

Article And Photos By: Will Ramsey Faith Forgotten Choppers www.faithforgotten.com

Originally Published In The August 2013 Issue Of Cycle Source Magazine

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A few weeks ago, I received a phone call from a builder in New York who wanted me to shorten a gas tank for him. He had chosen a tank with a shape that he liked, but it needed to be 5” shorter to properly fit his frame. I graciously accepted the job and when the tank arrived, I jumped right into chopping and redesigning the geometry. Now when you chop 5” off the back of a tank, you really have to think ahead about how you want to correct the shape. From the measurements, I knew that I would need to narrow the back of the tank 4”, however, instinct (and my eye) told me I would need to remove an inch from the front of the tank to keep it from looking like a balloon. So this month’s tech will discuss my solution to shortening a steel gas tank 5” while maintaining a balanced look.

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Here is the production tank that arrived at my shop. Now let’s get to work and trim this big girl down.

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I started out by cutting the tunnel with an abrasive cut off wheel.

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A hole saw is used to remove the stock filler bung. Since this will be welded back into the modified tank, it’s important to cut it out precisely.

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The cut line is marked to shorten the tank 5” and the cut off wheel continues chopping away.

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In order to narrow the back of the tank and keep the proportions balanced in the front, I decided to remove 4” from the back and 1” from the front of the tank. With some practice, masking tape can be used to layout very straight and accurate cut lines.

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These cut lines need to be precise to allow for a tight fit between the panels.

Photo top right: A tight fit along the weld line is absolutely essential to a well-built gas tank. Do not settle for the “I can fill it with weld” mentality. Take your time and strive to fit the panels tight enough that

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they can be tacked with fusion welds. This level of precision will allow for a consistent heat input along the weld seam resulting in more uniform shrinkage. In turn, the heat affected zone will be very easy to correct with simple planishing.

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The weld seam must be thoroughly cleaned with a solvent (acetone) to remove any oils which could contaminate the weld. Also, a clean wire brush (or Scotch-Brite) should be used after the solvent to remove any surface oxides which can also cause inclusions in the weld bead.

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Although it is technically not necessary when welding mild steel panels, I still prefer to use a back purge on all gas tank welds, ensuring maximum purity. This picture illustrates a backing tape that I use to capture argon on the back side of the weld. Again, this is not necessary but it is still an option.

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A 1/16th Tungsten and a 70S-6 .040 filler rod are used to weld the seam. I am careful to maintain a consistent deposit of filler rod along the weld seam. 1” of rod per 1” of torch travel allows me to lay the rod consistently, rather than feeding varying amounts of filler material.

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If the panels are fitted properly, and not pried into position with a screwdriver, it is not necessary to use stich weld (short welds used to limit warping). If the panels are not “stressed” then the only warping will be the weld shrinkage found evenly along the weld seam. Stitch welds increase the possibility for oxide inclusions in the weld bead. Take your time fitting the panels and use long, continuous welds. (Yes, I stopped twice on this tank to reposition myself).

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Since I maintained a consistent and minimal filler deposit while welding, I can now take the tank straight to the hammer without grinding material. Planishing a sheet metal weld is necessary for two reasons: first, the cold forging process of the hammer helps to refine the grain size of the weld, thus reducing stress and strengthening the seam; secondly, the stretching action of the hammer allows you to correct for the shrinkage which results from welding.

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As skills develop from lots of practice, the planishing process will result in a smooth even surface. Only a DA sander with 240 grit paper should be required to blend out any visual evidence of a weld seam. Making the weld seam disappear is not necessary and a strong argument can be made for not removing any material.

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A new hole must be cut in the tank in order to reuse the stock filler bung.

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A good tight fit allows for a nice even weld around the filler. Be sure to properly clean and prep the metal before welding!

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I usually bend tunnels in my press. However, since my press was occupied, I resorted to bending a piece of 16g cold roll over a piece of 2” PVC pipe.

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The new tunnel is placed in the tank shell and a trim line is marked.

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I use an electric shear to cut the 16 gauge tunnel. Again, precision is important to obtain a “no gap fit” between the tank and the tunnel.

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Once the tunnel is fitted and tacked into place, the desired mounting system can be installed. This customer wanted to upgrade to my hidden rubber mount kit. I marked an accurate cut line on the tank and was careful to position the mounts square to each other.

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Once the mounts were cut in and fitted, all final welding was performed. As usual, all weld seams were thoroughly cleaned with acetone and scrubbed bright with a clean, stainless steel brush. And again, I chose to back purge the tank prior to welding.

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The newly shortened tank is pressure tested for leaks, and ready to be shipped back to its owner. As always, if you have any questions or comments about this article please feel free to reach out to me.

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