73.5 911 Fuel tank lessons learned

[apex97]

I recently was fortunate enough to score a well preserved rust free 73.5 911T that has been sitting since 1984. Now when I say "rust-free" I mean the body has absolutely zero corrosion anywhere on it. I am happy, but its very unusual. Of course the fuel had been in the car for 30 years and it made a mess of the tank, lines and most of the CIS components. The tank on a 73.5 is unique because the cars had a full size spare but also had CIS. Tanks for later cars are being reproduced but I am not sure that they include the CIS "swirl pot" inside.
There are several threads about repairing and coating rusty old tanks and I thought I'd share some photos and my story about "going all the way" to repair this unique tank and why its no possible to properly clean and coat a CIS tank without cutting it in half and re-welding it.
The following posts will include some detail photos and explanations.
The first photos are both halves of the tank right after it was cut in half. It looks OK, but this was after it had been soaked with Vinegar for several days. There was still a bunch of 1/4" thick bricks of petrified fuel and rust trapped under and around the swirl pot. No amount of soaking could have removed it and the tank would never have fed fuel properly.

[apex97]

In order to get the tank perfectly clean and coated, the swirl pot MUST be removed. Let me give you the short version of why trying to coat your tank without doing this will destroy it.
The swirl pot is a small round plastic container about the size and shape of a quart paint can. It sits in the bottom of the tank, help down by three tabs. These tabs are not accessible without removing the lid from the pot. The lid is secured by three spring steel clips that can be carefully pried loose to remove the lid. (I wish I can taken photos of the inside of the pot!)
There are two fuel lines attached to the pot, a feed and return line. The fuel feed line draws from a small ring around the bottom of the tank. It is only 1/4" high and will be FULL of rust and dried fuel residue. The bottom plug in the tank is a screen filter that drains into this small round area to feed the pick up line. I found lots of junk under the swirl pot when I removed it. The pick up is not directly attached to the pot, but feeds from a metal circle that sticks up 1/4" from the bottom of the tank and protrudes up into the bottom of the swirl pot.
The return line is a steel line connected to a plastic fitting on the pot by a short length of rubber fuel line. The return line returns unused fuel from the CIS system directly into a channel in the pot where is swirls around the outside to remove any bubbles and then pours into the center of the pot to be picked up again. The lid has a hole in the top which allows fuel to flow in and fill the pot. Once the fuel level in the tank goes below the top of the pot, the pot is refilled only by the return line and a small alternate opening in the return line channel under the fitting. There is a very small opening in the vertical baffle that allows fuel to flow from one side of the tank to the other and feed the alternate opening. Without These small ports, you would run out of fuel once the level dropped below the top of the swirl pot because you would only fill it up from the returning unused fuel.
(This should be especially interesting for those doing a Carb conversion with no return line!)
If you tried to use a pour-in tank coating, you would inevitably trap material inside the swirl pot, blocking ports, openings and generally making it unusable. Also most coatings will not stick to plastic, so these chunks would slough off and float around the tank potentially causing issues.


The photo shows the two halves of the fuel tank, one before blasting and one after.




This photo shows the detail of the pick up and return lines that live under the swirl pot




This picture looks down the center of the swirl pot lid. Sorry I forgot to take pix with the lid off!



This is both halves cleaned up and ready to coat.



This is the coated bottom with the swirl pot re-
installed.




Coated bottom with Swirl Pot




Coated top.

Next step is to get it welded back together and repaint the outside of the tank to a clean stock appearance.

[E Sully]

Good luck with the tank. What did you coat it with, and what method will you use to weld it together. I would think standard MIG might burn away any coating at the seam, leading to rust on the interior of the seam. Heres my thread on the CIS tank.
CIS Gas Tank

[CountD]

Really nice work and documentation with all the photographs.

[apex97]

Originally Posted by E Sully

Good luck with the tank. What did you coat it with, and what method will you use to weld it together. I would think standard MIG might burn away any coating at the seam, leading to rust on the interior of the seam. Heres my thread on the CIS tank.
CIS Gas Tank

Your is a much better post than mine Ed! I mostly wanted to stress the challenges of any attempt to coat the tank without cutting it apart.

Tig welding will be used to weld it back together and we will remove coating on the back side of the welding areas before we weld. Make sure a pro level welder does he work or it will leak and never look perfect outside.

KBS Tank coating is what I used here.I have had great results with it on other products. Auto Gas Tank Sealer Kit - Gas Tank Sealer - KBS Coatings

[E Sully]

I hope the welding works out. You might be able to coat along the internal weld line afterward by carefully rotating the tank to avoid letting any sealer get to the bottom areas.

[porwolf]

Very impressive work, apex97 and E_Sully! I think opening the tank and thoroughly repairing and re-coating the inside is the best way to repair a CIS, or any other tank. It does not even seem very difficult except for the re-welding of the two halves. But any professional welder should be able to do that with not much difficulties and it should not be too expensive.

Can you guys also show us how you separated the two tank halves?

[E Sully]

Photos are from other threads I have found in my searches.
I have not cut my tank open, exterior has been stripped and repainted, but internally mine is still in good shape.