Here is a video of how-to fabricate a custom chopper gas tank top. The video is a little long and the sound quality isn’t that great, so you might want to fast forward a little. But its a good example of some basic metal shaping. I don’t know how good Baileigh tools are. I found this on their web site. It is a promo video for there multi-hammer. It looks cool. I believe you can actually buy a english wheel attachment for this thing too. That could be a bonus for anyone that has limit real estate in their shop. Any ways I though that it might give you a good idea of some basics on how to shape a gas tank for a motorcycle.

You can see that he starts out with what appears to be a wood buck. As he shapes the metal the buck is used to gage and check the shape of the gas tank. Wood is a great material for buck construction. It is easy to shape, relatively inexpensive and most “DIYers” have the tools you need to work this wood.

He starts by laying out a template with paper to get a flat pattern for the sheet metal. The paper pattern is a good way to plan out what needs to be done on any fabricated panel. If you lay the paper on the compound curve of the buck or body panel you can see where metal will need to shrink or stretch. After checking to see that the paper template fits on the buck he cuts the sheet profile with some electric shears. After he has his blank cut out. He use various type of dies or tools in this “multi-hammer”. He switches out the dies several times. The various types of dies and tools are typically used to do one of two things: stretch material or shrink it. That is really all you can do to metal besides cutting it and join it together (welding). Bending it is really just stretching it in a localized area. It a good idea to always keep in mind what you are doing. Are you trying to stretch the material or shrink it? What will that do to the shape of the panel? If you are always asking yourself those type of questions then you most likely will a avoid a lot of head aches and scrap less material.

I don’t think you can see in this video but the blank starts to get some waves or wrinkles at the edges as he works the panel into shape. That is because the metal in the middle of the gas tank is stretching to form the dome or crown of the gas tank while the metal at the edges wants to bunch together. A similar thing happens when you are making a pie crust. When you lay the pie crust dough in the tin you have to pinch the outside edges together so the crust will lay flat. This guy had to change out his tools to some shrinking dies so he could gather the metal to gather at the edges. If you were to measure the thickness of the sheet metal after he is done with the part you would find that the edges would be slightly thicker that the center of panel.

All you have to do in order to finish the tank is to keep doing the same thing with the other five sides and weld them all together.