Trimming the Fat Tech

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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.

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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
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.

Will Ramsey
Faith Forgotten Choppers
www.faithforgotten.com

3 thoughts on “Trimming the Fat Tech

  1. looks like you dont need a press or roller in your shop,you could do it all by hand.lol. real nice work on the tank!

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