build diary

March 15, 2006:
The rear end naked and ready for a new tank.
I'm going to put the tank in the deep area at the back and use the small hole in the center (you can see the grey sway bar in it) as a sump. It should work out pretty nicely.
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A new project, as if I needed one.
This is going to become a fairly potent naturally aspirated Miata rally car. More details to come!
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Maybe I should put one of these cars together so I can drive it!
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March 18, 2006:
I was supposed to be on my way to a track day, but a Colorado storm took care of that plan.
So instead I dug into the job of the fuel tank. Step 1 was to spend some time with cardboard. It looked fairly easy, but I had to consider removal as well as fuel flow and capacity. I do not want to end up with a tank that requires much disassembly to remove. After a couple of hours I had the solution and some quick calculations showed the capacity as 7.5 gallons. Excellent!

With a wave of my magic wand I transformed the cardboard models into steel. By "magic wand" I mean bandsaw, belt sander and welder of course. On the first test fit, the tank fit perfectly and I was able to install it without resorting to cheating. Hopefully that will be the case with the final unit! Once four sides were done I decided to weld up my sump. Once that was finished, I took a look and decided there was a better way. 30 seconds later, the plasma cutter had removed a big chunk of what I had. The sump is now complete and ready to be affixed to the bottom of the tank, then it's just a matter of a couple of internal baffles, the top and then sealing. POR-15 makes a gas tank restoration kit that should do the job nicely.
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March 19, 2006:
Today was supposed to be a ski day, but, well, it didn't happen.
So I went mountain biking and then attacked the tank again. I'm really enjoying this. The sump was welded on along with a couple of internal baffles and then the back. Now I just have to install the fitting for the return line and weld the top on the tank. Some sealant, paint on the outside and then it's just a bit of plumbing. I'm pretty happy with this upgrade so far. I think it's going to be all good with little or no downside other than a lost weekend spent fabricating. And is that really lost?
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The cardboard templates for the new fuel tank.
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There are some odd angles in the tank to let it fit into the car.
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The first test fit of the new "tank" - sort of.
It fits!
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The first sump.
It probably would have worked well, but I thought it could be better.
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The reconfigured sump, ready to weld on to the tank.
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The sump is in along with a couple of baffles to cut down on sloshing.
I'll have foam in the tank as well. The baffles don't go all the way to the top so I can actually put the foam in the tank.
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The back of tank is on.
Only one more panel left. Of course it will probably turn out to be the biggest hassle of the lot.
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Before I weld the top on, I think I'll add a couple of straps like this to keep the sump nicely filled.
Of course, in the current state I figure I won't have fuel starvation until I'm down to about a cup of fuel and pulling a high-g corner.
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March 21, 2006:
After cutting, sanding and welding all weekend I took the tank home to see how it fit in the car.
It slipped in with millimeters to spare past the roll bar but then wouldn't clear a tube at the bottom. The sump was hanging up. I peered underneath and discovered that a weld bead I'd run around the outside of the joint (it's fully welded inside) was hitting the tube. I ground that flat and the tank popped right in to place. It's as if it were designed for this car! Hang on...

I did a few happy dances as one did not seem to be enough. I could not be more proud of this tank. I've decided to put a couple of zip-tie anchor points on the outside to make it easier to hold hoses in place. I picked the sealant up yesterday as well. On the Locost mailing list, I was asked why not aluminum? Well, after paying for the Kreem sealant it's not cost. I don't know how to weld aluminum and I'm on a bit of a schedule to be ready for the Car and Driver test. But I do have to say that if I did it again, I'd certainly consider alloy. The finished tank is just under 16 lbs compared to about 8 lbs for the previous plastic one. That weight gain is about the difference between my new battery and the old so there's no real change overall. I won't have to run as much fuel to avoid starvation on the track so that's nice. Plus of course the weight distribution is much better.

The preparation for the magazine test continues. I've been told they're going to be doing instrumented testing so I'm nervous about that - what if the car only does a 0-60 run in 10 seconds? Well, a Caterham SV 160 does the sprint in 4.9, but that doesn't reassure me much. I've ordered some new springs, going to 500 lb in the front and 250 in the rear. Heikki thinks the 500 is far too soft but I think I can improve the wheel rate with some shock mount tweaking. He's also driving a pure track car and doesn't have sway bars. I don't feel the front on my car is too soft anyhow, it's the rear that needs beefing up. Will I regret this? Probably. But I'll have to make the mistake first. I can see Heikki shaking his head from here.
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We just got a plasma cutter at work.
I can't stop finding reasons to use it. Like this one.
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In the car! I could not be happier with the fit.
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A shot of the tank from the rear.
The sump is quite obvious here.
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The sump from the side.
The fuel line is just hanging off here as I had been checking the clearance for the nuts. The ugly welding along the seams is not needed, it's fully welded inside.
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March 22, 2006:
I used a smoke machine to check for leaks last night.
It runs a couple of psi of pressure and is great for finding air leaks. A couple showed up and I decided to weld all the seams in the tank from the outside as well as the inside just to be sure. That stopped the smoke leaks, but when I filled the tank with methyl ethyl ketone to degrease it I found a couple more. I've decided to grind all the exterior welds down to check them for pinholes and also because, well, it'll look prettier! With the tight fit in the car it's likely any external welds will cause installation problems anyhow. The coating should stop the pinholes but I'd rather use that as a backup instead of relying on it. The current plan is to finish that grinding and cleanup tonight, then coat the tank tomorrow.
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Using a smoke machine to check for leaks.
I found a couple. This picture was taken after I was done - you can't test with the gas cap off!
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March 23, 2006:
Last night I spent a bit more time fixing little leaks.
Not squirters but slow drips. The sealant would take of that but I'd feel better having metal to back it up. Then I degreased the tank with methyl ethyl ketone (MEK) and filled it full of Kreem Tank Prep A. From the smell of it and from my reading, that's basically just dilute phosphoric acid. So is Metal Etch, so I used part of a bottle of that to get to the desired concentration. I left the tank to stew overnight - longer than required for a new tank, but the timing worked out for me.

This morning I came in early to discover a couple of spots where the acid solution had escaped past the tape and made a nice mess on the workbench despite a bunch of prophylactic paper towels. Ah, let's just call it art. I rinsed out the tank and used Kreem Tank Prep B (aka MEK) to get rid of any water. Then it was time to play with the Kreem goo itself. Pour it in, rotate the tank all over to let it coat, let it sit, rotate the tank all over, drain and let dry. The second coat will go on in a couple of hours and then I'll let it cure for a few days as I play with plumbing and painting the outside. Then, finally, I'll have the tank done.
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Mmmm, chemicals.
This is what it takes to seal up a gas tank. The aluminum tape in particular is very useful. Also note the attractive finish on the workbench after a small acid leak.
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