I replaced the fuel tank in my Dart a month or so ago (it was half the price of a new tank just to get my old one professionally cleaned), and installed it with a brand new sending unit. I remember that when I installed the sending unit it wasn't super tight, but it seemed tight enough to seal.

Well, yesterday I finally filled the tank up high enough to notice the obvious leak. Brand new tank, sender, and gasket, so I don't know why it wasn't tight enough. I believe I have an extra gasket and lock ring, as both the tank and sender came with them. I'm thinking I will try to double up on the gaskets, but this might be too thick to close the locking ring.

If that doesn't work, does anyone know where I can find a slightly thicker gasket? Or recommend some material for cutting my own?

Also, sanity check: the gasket goes between the sender and tank, right? Not between the locking ring and sender? Maybe I could put a gasket on both sides of the sender...

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

the gasket goes between the sender and tank. I just recently replaced my tank,sender & strap.

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1964 Dart GT

What tank did you use? I replaced my old tank with a Spectra tank with the sending unit, so far so good on the seal no leaks and this is my third or fourth tank of gas. I posted a few pics over in the gallery. One thing to make sure of is that your seal stays pretty much centered with the hole and the sending unit for a good seal. One seal should do it........

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1964 Dodge Dart 270 four door. Three speed automatic 225 Slant with a Carter BBS carb.

I don't recall who made the tank or the sender. They were both Rock Auto parts.

Two gaskets together were too thick to get the locking ring installed. I ended up taking off the locking ring and bending the little locking tabs outward a bit with a pair of vice grips. It was harder to get the ring installed again, but now the ring presses tighter against the tank when locked in place. I filled it back up and don't see any leaks yet. I believe a gasket about 1.5 times as thick as the stock one also would have worked.

You're right about keeping the gasket centered, and the lock ring too. The hardest part of reinstalling the lock ring was keeping the opposite side from popping out when I would hammer one side. The lock ring's outside diameter should be a bit larger, I think. To keep it centered during installation, I cut a few short pieces of small rubber hose and slipped them over the edge of the lock ring, as shown below.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Looks good glad it sealed, looks good I have a spare seal and ring if you should need them.....

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1964 Dodge Dart 270 four door. Three speed automatic 225 Slant with a Carter BBS carb.