I need to flush the gas tank for my 70 Supreme.Tank is out of the car.I took the pick up assy out today and the tank is very clean inside.There is maybe a cup or two of really old tarnished gas and I can hear something that sounds like a couple of small rocks rolling around when I move the tank around.The gas cap is on and sealed with tape so what ever is in the tank must have gotten in through the vac/fuel lines. TIA

 

The small rocks rolling around are probably small pieces of rust that have broken off. It's doubtful actual rocks got into the tank unless someone deliberately put them in through the filler neck.

When I had the tank off of my '67 Delta 88, I had the same issue. Small pieces of crud rolling around in an otherwise clean tank. I found that the opening for the sending unit was just large enough for me to get most of my arm in there. I just tried to slide the crud toward the center of the tank as much as possible by tipping it back and forth and then reached in with a damp paper towel and wiped stuff up. I did that a few times and got pretty much everything out. Recognize that you probably won't get 100% of the crud out, but getting 95% of it will do just fine.

 

As long as it is off, I would take it to a radiator shop and have it cleaned and sealed. Then you are done.

 

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I haven't tried this myself, but an old mechanic I know said he dumps paint thinner and some chain in the tank, then rolls it around a bit and empties it. He claims it gets a lot of the crud out. Personally, I'd rather find a radiator shop and have it boiled out instead. Whatever you do make sure you empty out the old gas first, then flush it with lots of water.

 

Cleaned and sealed or you can do it yourself. I believe eastwood sells a good quality sealer that you can use to remove the varnish/rust and then seal the tank. Pretty easy and the kit is inexpensive compared to a new tank

 

Looked around for a radiator shop yesterday, believe it or not there is not a single rad shop in the yellow pages.I may flush it with the parts washer at work then take it to the car wash after.

 

I know the big cars don't have it so easy, but replacement tanks for a 70 Cutlass are readily available for around $150. Something to think about if the original tank is pretty rough.