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May 31, 2016

Fuel Tank

[Click the pics for a better view]

TR6 cars had a formed steel fuel tank with a capacity of 10 to 12 gallons or so, depending on the year, and maybe the market.  The tank is situated under the rear deck, between the passenger cabin and the boot.

My tank was dirty with some rust here and there, but appeared outwardly to be intact.

The fuel level sender had a lot of RTV-type stuff smeared around it, maybe from the factory.  

Inverting the tank yielded a surprise--along with the expected dusty and scaly material, out came a loose sender float, a third full of liquid.  This tank hasn't had anything liquid in it for at least 25 years.

The brown material looks superficially like rust, and some of it certainly is, but closer inspection suggests that it's mostly just dried fine or sandy textured sediment.  A peek inside the tank revealed more of the material, but also areas that looked like bare metal with some surface rust.

I was encouraged enough with the condition of the tank to spend a little more time cleaning it up.  I've done a couple of motorcycle tanks before, and I take a very systematic approach to cleaning.  To get the gross crusty stuff out of the tank, I use mechanical means.  I put heavyish abrasive material in the tank with some soap and water, and agitate for a while.  Though there is actual media made for this purpose, many people use nuts and bolts or drywall screws for the abrasive.  I chose to use a few pounds of small jagged rocks.

Now with a motorcycle tank, manual agitation isn't too bad, but after a few minutes of shaking the TR6 tank back and forth, I decided this would be an excellent application for some sort of automation.  I took a break to mull it over.  I wandered to the shed and had one of those AHA! moments when my eyes locked on my old cement mixer.

A little disassembly of the mixer, a circular plywood platform, some hardware, and--Automation!

Before hitting the on switch, I of course had to seal up the openings in the tank.

With all systems go, I launched.  It saved me a lot of effort, but Chippie the dog didn't approve at all.

Everything was cool for the first 10 or 15 minutes of rotation, but then I saw something I didn't want to see--some drips on the floor.  Hoping it was just a leak from one of the plugged orifices, I stopped the contraption.  It was what I feared, a small seep, probably from a pin hole.

It was a very slow leak, so I went ahead and finished 30 minutes of rotation, then flipped the tank and did 30 more.

With this new discouraging information, I decided to sand blast the outside of the tank to see how bad it was.  There was a tight little group of pin holes at one place on the bottom of the tank.  

The pitting suggested to me that the rust through was from the outside, but later internal inspection would confirm that.  Since I'm not a fan of internal tank coatings, and because the damage was from outside the tank, I decided on an external fix.  I applied a thick coating of lead (solder, actually) to the pitted area on the tank bottom.

Then, back to the tank cleaning regimen.  After physically removing all the loose material, the inside of the tank still had a smooth reddish brown surface coating.  This was just gasoline varnish residue, either a non volatile fraction of the gas, or some decomposition product.  It's the same stuff that gums up carburetors when they sit for a long time and the fuel evaporates.  Being organic, an organic solvent is needed to remove it.  And wimpy solvents won't usually do it.  Carb cleaner is a pretty strong solvent usually made of a mix of acetone and toluene and often a few other things.  A cheaper solvent that has a similar composition is lacquer thinner, so the next step in my cleaning process is an automated slosh with half a gallon of lacquer thinner.

After the organic solvent step, the inside of the tank was bare clean metal, but there were a few areas with some rust.  Since organic solvents don't touch inorganic rust, a different attack is needed.  Out comes the lacquer thinner (reddish brown now, due to the dissolved varnish), and in goes a gallon of phosphoric acid solution.  Phosphoric acid dissolves rust, and also leaves a thin coating of grayish iron phosphate that offers some limited protection against corrosion.  

Here is the inside of the tank after the acid.  The little stub at the back near the top of the picture is the tank outlet.  Notice how it sticks up a half inch or more above the tank bottom.  This is to try to keep any sediment from getting into the feed pipe.  The silvery areas near the center of the picture is where the solder came through the pin holes.  The orange specs are left over chips of the rocks used in the first step.  Those came out pretty easily after everything dried.

For those into masochistic self-flagellation, here are links to some shaky borescope videos of the inside of the tank after each step:

    vid1--After the loose material was dumped out.
    vid2--After the mechanical cleaning and pin hole repair.   Shows the varnish coating.
    vid3--After solvent cleaning.
    vid4--After phosphoric acid.

At this point, the tank was liquid tight, and the inside looked pretty good, so I decided to keep and use the tank.  I gave it a couple of coats of POR15 to finish the outside.

The tank had a couple of foam strips glued to one side that didn't survive being removed, so I made some new ones from some sheet foam of similar properties.  Some suppliers offer felt strips for this application, but mine were definitely foam.

The original sender was pretty gross looking, but still worked.  I could have cleaned it up, but was concerned about the wear on the wires.  A wiper rubs on a winding of resistance wire, and I could see that the wire was definitely scored under the path of the wiper.  The new sender I ordered looked very close to the original, and had similar resistance characteristics, so I decided to use it.

Installed the sender with good coating of Permatex 3D on both sides of the gasket.

Then applied a new foam pad around the outlet tube.  I'm just lacking the little ferrule to make a new tube.

I can't wait for the day I slide this dude home.

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