I decided to give making an aluminum fuel tank a try, out of interest rather than necessity. The sole advantage is that it was easier for me to make the bungs from aluminum than from steel. The tank was first designed from drawings of the components - fuel pump, filler and sender. The parts were then fit to a tank that would also fit in the space left between the W's. The resulting design provided a tank of approximately 35L capacity. The drawings were printed full size on a bubble jet printer, sheets taped together then cut out to be used as templates for the aluminum blanks. You can get the drawings from the downloads section.
First I built a hammer form cut from MDF, then shaped with a router using a 3/8" round over bit. A second, smaller MDF version of the shape is used to hold the aluminum in place against the hammer form.
The next task was to cut the blanks from the aluminum. The end and baffle blanks where cut, then formed over the hammer form. I trimmed triangular cutouts from the corners from the end blanks to help them follow the shape of the form. I presume they could be shrunk to fit, but I wasn't able to manage it, and since I was welding them anyway, didn't see the need - the cut out would be welded along with the tank ends themselves. The baffles were also shaped over the same former, with the intention of bending them slightly more to fit inside the tank.
Most important thing before cutting the sheet for the tank body is to bend a test strip to determine the exact dimensions and bend points needed to shape your tank correctly. Do this by checking it against the hammerform shape, which should fit snug inside the correctly shaped tank.
My design required a single sheet for the tank body of 33" x 30 1/4". I bent it up in my home made sheet metal brake. The brake itself scratched the tank, some sanding of the brake to smooth it helped a bit, I'll have to work on that more later.
Bungs to hold the components in place must be drilled and tapped for the fixing screws, and the centers removed to clear the components. I used one 3" diameter bung for the tank sender and two 5" diameter bungs for the fuel pump and filler. The bungs were drilled out with inexpensive wood hole saws. Slow, but adequate for aluminum, bimetal blades would be needed for mild steel.
I gas welded the tank together using what I learned from the Tin Man, including using the special aluminum welding flux which is an absolute must to do this successfully.
Clearly, from the shots, my welds are not pretty, but they got better as I went along, and they appear sound.
I made some mistakes in the manufacturing. First, I deliberately left the tank body blank 1/2" too long based on the test strips, thinking it would be easier to hold the shape while tacking it together. I think it would have been just as easy to hold it in place with clamps, and easier to weld the edges together if they fit flush - I found it was much easier to weld the tank ends in place, which did fit flush. The second mistake was with the baffles. My plan was to weld them in place and use them to hold the tank shape while I welded it together. This resulted in some distortion, I think it would have been better to tack the tank body first, then weld in the baffles. The third was welding in the bungs after the ends and baffles were in place - I think the bungs should be welded in place after tacking together the body, allowing any distortion to be hammered out before continuing.
From the metal shapers group on yahoo, I read that it would probably be better to either rivet the baffles in place and weld the rivets (use ali rivets!), or drill through the tank body and plug weld to the baffle flanges.
After tacking then welding the tank body, then finishing the baffles, I cleaned up the tank and then welded on the tank ends. Next, the bungs were welded in place. To weld in the bungs, I first found the correct location on the tank, and drilled out the matching hole from the tank body. This allowed me to use c-clamps placed through the hole to hold the bung in place. Welding the bungs in caused considerable distortion to the tank, which could not be corrected since the tank ends were already in place. I tested the tank by installing the fittings, inserting an air hose at low pressure, and plugging with a wet rag. Soapy water was used to check for leaks, and any found were rewelded shut.
Unhappy with my aluminium welding, I decided to build a second tank, using the same design, from 18g steel. This one worked out much nicer. A second change was to use appropriate sized sealed style rivet nuts rather than a welded bung to hold the tank components. A few shots of the finished tank are shown below. 18g strips cut to fit the original rubber tank mounting strips are used to hold the tank in place, again using appropriate placed rivet nuts to hold them in tightly around the tank.
