The Great Fuel Tank Challenge
This fuel tank project has been the most ambitious part of my build so far. I grew up playing with the welding gear in Pops Racer's shop, and I did cobble together some toys over the years, but never developed anything like what a real welder would call "skills." More recently I've picked up the gas torch again and have gotten pretty serviceable with brazing steel. But welding aluminum is notoriously difficult. At the suggestion of Pops and a friend of his who is an even more accomplished metalsmith, I started practicing with oxy-acetylene and scrap pieces of 3003 aluminum. After a lot of work, I got to the point that I could make an intact weld for about 2 inches before burning a huge hole in the metal. The real benefit of all this practice was that when I went back to TIG, my welding had improved dramatically. So I uploaded my CAD files to be cut and bent by Send-Cut-Send.
The picture above illustrates the loveliest delight of ordering laser-cut CAD parts: everything fits. In this picture I have all top front and bottom pieces all temporarily attached to the middle baffle with skin clips. The right side of the tank is facing up here. The way these parts all line up so nicely would not have been achievable if I had cut and bent all these parts myself.
Next came a bunch of solid rivet work. Here I'm riveting the filler neck flange to the inside of the tank. This flange is, as Grandma Jones used to say, "boughten:" purchased from the same place I got the filler neck itself. Grandma Jones made so much of her own everything she had to have a special word for anything she didn't make herself.
These flanges - for the OEM Mazda fuel pump assembly and vapor vent cap - aren't boughten. The fuel pump assembly flange can be had from a place in England but it's pretty pricey and it was not a terribly difficult job to draw it up in CAD and have it laser cut. If you ever do this beware: The hole pattern is NOT symmetrical. It was $9 for the material and the cutting for the upper fuel pump assembly flange. Note that both of these flanges have a matching piece on the inside of the tank, tapped for m4 screws. Again these flanges needed to be flush riveted in place before welding the parts together.
The line of three "dot welds" is where I welded over the rivets that hold the baffle in place. These extend all the way around the tank. If not welded over, the rivets will leak.
The next job was to tack weld all the pieces together. Aluminum loves to warp so you have to make a large number of tacks on all the joints all around the tank - then weld the spaces in between the tacks. My welding improved during this process so the general pattern is about two inches of reasonably competent looking welding, then a messy blob where I did my tack weld, then another two inches of semi-decent, repeat all the way around.
There were some tricky bits at the corners. When you bend aluminum, it wants to crack in the corners - so you have to make relief cuts in the sheet metal. In the picture above left, the nearer (right side) relief cut isn't too difficult to manage - a gap like this can be welded over with minimum usage of swear words. But the inside corner on the left of the left side picture has a larger gap because when you bend a chamfer around an inside corner there is not enough material to close the gap. This required welding in a "patch" as seen in the above right picture. These patches were easily the most difficult part of this entire welding job. I will not comment on the swear word usage during this part of the job. When my son Byron was a kid and we went to the shop, I always reminded him not to use Engineering Language when we got back home.
So here is the completed fuel tank sitting proudly on Pops Racer's welding bench.
Above is far and away my favorite picture from this entire project: a successful Dunk Test. I installed all the fittings and capped off the openings and immersed the tank in one of the horse troughs. Pops' shop is surrounded by Amom's farm, so there are handy nearby facilities for dunk testing. It took both hands to push the thing underwater but the key finding was NO BUBBLES. I brought a towel and a marker in anticipation of marking all the spots where the tank was leaking, then going back to the welding bench to fix them. Astonishingly, the towel and the marker went unused.
Here's a project in which I did not cover myself in glory. I am making the trunk floor out of marine grade plywood, reinforced with fiberglass and fully coated in epoxy. This is the underside. The box at the upper right is a recess for the battery. The cutout just to the left of the battery recess is where the fuel tank goes. Now here's the D'oh moment: one of the primary reasons I made this odd-shaped fuel tank is that there are underlying frame structures in the way of anything dropping below the front part of the trunk floor. So having gone to all this effort working around those frame rails, I foolishly made a battery recess just exactly where the dang frame rail is. Of course I didn't discover this until I'd epoxied and fiberglassed the whole thing together, and at that point there was no taking it apart intact. So now Trunk Floor Version Two is underway in the garage.
I wasn't delighted with the packaging of the OEM Miata fuel filter. Family friend Paul, who is an Actual Real-Life Mechanic who Actually Knows What He's Doing, suggested this Bosch unit (used in a lot of VW and BMW products). This meant making a mounting bracket. I found this exhaust pipe clamp that fits nicely around the filter and has two bolts for bolting on to something, so all I had to do was fabricate the something: the aluminum bracket seen in the lower part of the left hand picture above. Right side picture shows it assembled. After welding the tank, my hard-won aluminum TIG skills are such that this little project was a breeze. I will hang this from the Replicar frame, in the trunk, just to the right side of the fuel tank.
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