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Fabricating a bigger gas tank Print E-mail

Fabricating a BIG gas tank




Dave Hansen, an XS-1100 club member and a professional associate at the Battelle Pacific Northwest National Laboratory in Richland, Washington saw my Big Gas Tank (BGT) last April ('96) when I had mounted it on my XS-850G in preparation for a trip to L.A. At the time, he told me that other XS-1100 members might be interested in hearing how I made it. He suggested that I circulate the write-up via the Internet. Until recently, I was having trouble with my ISP and was not able to get the word out. Now I can.


I am an engineer and a life-long motorcycle enthusiast. Wanting to be a motorcycle engineer, I chose mechanical over electrical engineering (even having a start in the latter field as an electronic technician in the A.F.). Ideally, I wanted to work for Honda. That didn't happen, but my first job out of school was with Harley in Milwaukee as test engineer. That was a fun job, but I really wasn't a Harley kinda guy and I didn't like Milwaukee's long, cold winters.

BGT Concepts:

I generally liked the idea of a bigger gas tank, about twice the capacity of most stock tanks, primarily for touring. Maybe I wouldn't want to sit on the bike as long as the BGT would allow, but at least I would have the option to, and also the option to pass up high priced gas stations. I considered various construction techniques. I wanted to maintain secure, stock tank-like mounting and retain adequate clearance under the tank for the "stuff under the tank", to use stock filler cap and petcocks and maintain a comfortable width between the knees. I wanted to keep the stock tank for non touring riding and planned to mount the BGT when I prepared for a trip..

While attempting to ride trials with Pacific International Trials Society (PITS) in Northern California, I had occasion to have the fiberglass tank-body of my '71 Montessa Cota repaired, it having been bashed onto the rocks a few too many times. I knew I didn't have the high degree of expertise that was obviously required to do this work so I had a fellow PITS member do it for me. I learned from this experience, that fiberglass work wasn't Rocket Science, and even I could do it. I first proved my fiberglass expertise by using this technology to reinforce a Japanese aftermarket tank body for the Honda TL-125 I modified for my M.E. senior project.


To make my BGT-1, I decided to take an existing, stock tank, cut away un-needed sheet metal, keeping the tank tunnel, mounts, floor with petcock and the filler cap/ flange then bond onto it a bigger fiberglass tank shell. BGT-1 was intended for one of the bikes I had while in Milwaukee, a '71 Honda CB-500 four. I aimed to about double the stock 3.6 gal. capacity.

BGT-1 Fabrication Steps:

I obtained a dented CB-500 tank and removed the flip top filler cap and latch parts and the petcock. Then, using a combination of saber saw with a fine metal cutting blade and sheet metal snips, I first cut out the filler cap flange area, leaving about 1" (25mm) around it for subsequent bonding to my fiberglass tank. Then, I cut away all of the tank except for the tunnel with mounts and floor with petcock boss. The remaining tunnel/ shell would still mount onto the bike as the completed tank eventually would.

The new fiberglass shell was made by laying it up on a male mold I fabricated. I chose this method because it seemed most straightforward for a one-off tank. For BGT-1, I stacked cut and tried Styrofoam blocks on the cut-up tank's floor and over the tunnel and glued the together with construction adhesive. I then carved the glued-together blocks with a coarse rasp till I had the shape I thought I wanted, minus about 1/4" all around. That 1/4" would be taken up by the fiberglass I would laminate. I did this in the basement of my Milwaukee flat but still had Styrofoam fuzzies tracked all over my place. I wasn't married then, so it didn't matter.

I made sure I had plenty of clearance for forks, triple clamps and handlebars lock-to lock. I retained the stock distance between the knees with knee notches, forward of which, the tank widened out about 2" on each side. I kept the overall tank width to be about equal to the width of the engine and made the mold about 1.5 times as tall as stock. I provided about a 1/8" recess in the top to accept the filler cap flange.

I filled and smoothed the surface of the mold, as necessary, using Bondo. Since Styrofoam is readily dissolved by the polyester resin used in fiberglass work, my mold needed to be protected from the resin. I brushed melted paraffin wax onto the top and side surfaces in an attempt to keep the resin away from the Styrofoam. I also sprayed mold release onto the wax to assure easy separation of my laminated tank shell from the mold.

The mold now defines the new extent of the new tank flat floor. I taped cardboard onto the portion of the remaining steel floor to serve as a pattern. With the mold positioned on the tunnel, I drew the outline of the edge of the mold onto the cardboard. I then offset this outline by about 1/8", outside, to allow for the thickness of the tank/shell. This offset pattern served to layout and cut sheet steel (~.02", .5mm thick) floor extensions. I brazed these extensions onto the remaining tank floor maintaining the alignment of the patterns as marked and cut. Care must be taken during the brazing process so the steel tunnel/floor and new floor pieces are not warped excessively. I made the floor of BGT-1, essentially flat, extending flat toward the front, where the stock tank floor curved up. The intended style was something like that of certain classic British irons like the Norton Manx.

To help the securing and bonding of the new shell I would lay up, I cut and bent up some angle pieces of sheet steel, with a cross section of about 1/4" by 1/2" (6mm by 13mm). I brazed sections of these onto the extended floor at places I expected to need the extra support so that they would be flush with the inside of my tank/shell. I can tell right now, that I need graphics to properly illustrate this write-up.

Now I began to lay up the fiberglass. The process is to brush a thick coat of laminating resin (laminating resin has wax in it, the wax rises to the surface of the resin when applied and keeps the surface tacky to enable good bonding to the next layer) onto the male mold, cut, as necessary, pieces of glass cloth and lay them into the resin and brush more laminating resin onto the cloth so it's saturated. Be sure that the cloth lays down onto all of the surfaces and there are no bubbles underneath. As soon as the resin has hardened (an hour or so) repeat the process.

I guessed that 2 laminations would be rigid enough to maintain the mold shape so I popped the tank shell off the mold at that point. I was right about the number of laminations but found out that some resin had gotten to the Styrofoam and partially dissolved it, causing the mold to cave in a little bit in a few locations. This was not so severe as to keep me from proceeding, though.

I positioned the shell over my extended-floor tank tunnel and it almost fit pretty good! With the shell in place, I drilled through the shell and the brazed-on angle pieces, at several places on each side, with a drill size suitable to accept a small sheet metal screw. I countersunk the drilled holes in the shell to fit the flat head screws I intended to use. I coated mating surfaces of the shell and extended floor and angles with laminating resin and assembled the two parts, securing with the sheet metal screws.

At this point I glued the filler cap flange into the recess in the shell left by the recess in the mold with laminating resin.

Now, I began to laminate more layers of glass cloth and laminating resin, making sure that I laminated fully around the edge from the sides to the floor, extending the cloth a few inches onto the bottom of the floor and onto the top of the filler cap flange for a good, strong bond. Some sanding, between lamination layers was necessary to smooth out bumps. In all, I laid up about 6 layers of the medium weight glass cloth, providing a wall thickness of about 1/4" (6~7mm). This was to be a strong, non-leaking tank.

I put on one final coat of sanding resin (no wax, so it sets up hard). The tank required considerable sanding and smoothing with Bondo to produce a smooth finish. I primed it with automotive spray can lacquer, sanded, primed again till it looked right and painted it fire engine red with automotive spray can lacquer.

One touch was to mask, from priming and painting, a vertical strip about 1/2" (13mm) wide by most of the height of the tank. This left the semi-transparent, greenish fiberglass shell visible as a fuel gauge. I calibrated this by incrementally filling the tank and marking the incremental levels next to the gage in pencil. I finished this by marking these calibrations with a scale and ticks and labels by means of a rub-on lettering set intended for marking electronic equipment. I sealed the marking with a carefully applied layed of clear lacquer over the gage and lettering. The gas gauge turned out not to be visible unless you got your eye right down next to it, difficult to safely do while riding.

I used BGT-1 on several trips and in the Arab gasoline embargos of the '70s. The automotive lacquer held up well and remained quite appealing. I never had any problems with leaking or structural failure. I never crashed on it, either. I sold BGT-1 separately from the bike in about '79 to a guy in L.A. (where I lived at the time) who wanted to use it on his CB-550 which he used as a small parcel delivery vehicle.


In '78, I bought a new Yamaha XS-750E. The CB-500 was physically too small for two-up, luggage-loaded long distance touring. Soon I was envisioning a BGT for it. Again, wanted about twice the stock capacity. Now, I was going from 4.5 gallons. I realized that, because of the greater capacity, I would have to sacrifice aesthetics and styling more than with BGT-1.

I used essentially the same technique as with BGT-1 except:

used balsa wood blocks to make the male mold, to be attacked by the resin proof.

retained part of the stock steel tank sides adjacent to the knees, I liked the feel.

moved the filler cap to the right to reduce spillage when full & on the side stand.

used Yamaha spray can paint (Carmine Red), never did work well on the tank.

I built this one in the garage (and at work), I was married now & my wife didn't like fuzzies in the house (she doesn't like metal chips or grease, either).

While building this tank, I was working for McCulloch Corp. (the maker of those little yellow chain saws), In L.A. I brought the balsa block mold in several times to use the shop's wood pattern making power tools. The engineers and techs In the lab began to speculate on the ultimate capacity of my "Super Tanker", as they called it. They set up a pool to wager on the capacity. To make it "fair & legal", official McCulloch Corp. dynamometer gasoline was poured into the finished tank to measure the capacity. Capacity was determined by the assumed density of gasoline at that temperature and the difference between full and empty weights of the tank. The calculations indicated 9.0 gallons, the 2 X OEM capacity I had strived for. A visiting representative of Underwriters Laboratories, who was certifying that our chain saws were safe, won the pool. I won a full tank of McCulloch gasoline, after all, you couldn't use it for chain saw testing after it had been tainted!

After the fact, I found that I had not really left adequate clearance between BGT-2 and the front forks and triple clamps and for the throttle cable "L" bend fitting where it exits from the twistgrip, when the forks are full lock to the right. None of these problems were catastrophic, but some bumping and scratching of the fiberglass did and still is occurring. Also BGT-2 fits a bit too snugly inside my Wind Jammer knock-off faring. The tank and faring rub on each other a bit. As with BGT-1, I have had no structural problems or leakage, from the tank. Leakage around the vacuum diaphragm peacocks is another story!

I have used this tank on several trips, but prefer to run the stock tank around town. The full tank is obviously heavy and restricts handling responsiveness somewhat. Straight and level, on the freeway, it is of no notice. It is a good, flat, stable place to mount a tank bag. I still retain BGT-2 for future long distance rides. After-the-fact, I see that I should have paid more attention to clearance, up front and on the front sides.


I have a general plan to build another tank for my Yamaha SR-500, using proven BGT technology. I don't know when I'll get around to it, but, sometime... My plan is to make a more stylistic tank, more cafe' racer-esque, suitable for the 500 thumper. I envision the Manx look with flat bottom up to the front, with the front somewhat spherical, curving up and back, faring into the slightly raised top. Of course the forward portion would be suitably wider and comfortable notches would fit my knees. I would lengthen it to the rear, a few inches, encroaching into a portion of the seat I don't use for solo riding, anyway. I would not sacrifice aesthetics for capacity with this one!


This approach to building a custom/bigger motorcycle gas tank appears to be effective. It requires a bit of effort, but I believe is within the capacity of most enthusiasts who maintain their own bikes. The majority of the work is somewhat tedious, step-by-step attention to detail (if one expects to have a functional, safe, and aesthetic tank resulting from the effort). About the only step that many individuals may not be equipped for is the sheet metal brazing. Once the metal has been cut out and fitted, it would be straight-forward for a local welding shop or friend with the equipment to do it, at low cost.

I stress that considerable care and consideration must be given to assure a strong, leak-free tank: after all you are working with the part of the fuel system which carries the fuel, even more fuel if you expand the capacity. If I were a lawyer, I probably wouldn't even have written and disseminated this. However, I am a motorcycle enthusiast and just want to let others how a bigger, custom gas tank can be built.


I certainly did a poor job of photographically documenting my BGTs. The two photos and one CAD sketch will have to do!

This photo shows that BGT1 looked pretty good on that bike:


This photo of my daughter, Franny, on my XS-750E, at least, indicates BGT2's width. It measures 15" (38cm) from point-to-point across the top in front of the knee notches. Also the fitment of the OEM flip top gas cap is shown. The poor paint job is evident.


I drew this Cad Sketch after realizing that my written ramblings and hand waving might need a bit of graphical backup:




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