Tuesday, September 3, 2013

Gas Tank Fabrication (Part 1 of 4)

I am honored to have been asked by Chris to write a tech article for Cycle Source Magazine this month… I decided to write about my favorite part of building a bike - fabricating the gas tank.  Although most of this tech will demonstrate the use of an air power hammer to shape the metal, there is nothing that I do with the hammer that cannot be done with hand tools. I have built many tanks with hand tools only and the principles are all the same.  The hammer just allows me to do it faster… The quality of the finished product is simply a result of the time and effort you put into it.
This will be a 4 part tech article. 
Part one is focusing on design, and metal shaping.
Part two - welding, planishing, and metal finishing weld seams. 
Part three - finishing the tank and mounting.
Part 4 – pressure testing and sealing the tank.
I hope you find something of interest in this Tech… Please feel free to call me as I love talking about this stuff. 

I start off by sketching and cutting out a profile template of the tank.   I’m always trying to visualize the 3D shape in my head…

After tracing the template on a piece of 18gauge mild steel, I draw a second border ½” larger using a set of dividers and then cut along the outside line with a Beverly Shear…

I’ll start shaping the metal by shrinking the edge. Here I have marked the areas that will require the greatest amount of compressive shrinking…

Using a set of shrinking dies in my Air Power Hammer, I begin shrinking the edge of the panel. These dies allow me to maintain the form of the panel while shrinking… (Remember this can all be done with a tucking fork and a mallet… Just takes longer)

I switch to steel hammer dies in order to raise a crown in the center of the panel while blending and planishing the areas I have just shrunk.  

The English wheel can also be used to raise a crown and blend into the shrink. However, I tend to use it primarily just to smooth and qualify the surface after using the hammer.

I gently use the Erco along the top and bottom of the panel to create more curve from front to back. This can also be done with the Lancaster style hand shrinkers.

After laying out a line along the bottom of the panel, I return to the shrinking dies in order to roll the bottom edge.  

If necessary a hammer and a dolly, with the proper radius, can be used to true up the rolled edge.

All of these steps are performed equally to the left and right panel in order to maintain symmetry. 

After positioning the sides of the tank I’ll use tracing paper, magnets, and charcoal to trace out a template of the tank top. 

Using the “body” dolly, I’ll form the basic contour of the top.

Again I’m back to the shrinking dies to shrink the edge of the top panel.

This time, due to the overall curve of this panel, I need to use the planishing hammer to raise the center and smooth the surface.

Once I have a good tight fit, I’ll use some layout dye and a scribe to mark a trim line for the top panel.  It is critical that the panels fit tight so that we don’t run into issues during the welding process.

After thoroughly cleaning the panels with acetone I’ll begin tacking the edges together with the tig.

I cannot stress enough the importance of perfectly lining up the weld seam before welding. I’ll work the seam from above (hammer and dolly) or below in order to maintain alignment.

Check back next month for welding, planishing, and metal finishing the weld seams…

Will Ramsey
Faith Forgotten Choppers


  1. Looking forward to part 2

  2. I always wondered how pressure tanks were made. I feel like when you're dealing with pressure, there are so many things that can go wrong! What if a valve isn't closed all the way? That could end very badly.