I installed an ebay 3/8 sending unit and now I have a leak between the tank and the sending unit. I took the thin cheap locking ring off and put the original thicker one on but it still leaks. is there a sealant or glue I can use on the rubber gasket that will help it seal or am I doing something wrong? it only leaks when the tank is almost full.

Same tank & no leaks w the one you took out, just upgraded to a 3/8"? I'd compare the new one to the old(flatness/diameter)& the thickness of the flat metal(some are cheap) & might get or cut a new neoprene gasket possibly one a bit thicker.

I still have the old gasket do you think it would be worth trying to double them? good to hear from you again Rapid

Sorry I missed the first question on your reply. Yes it's the same tank as for leaks I can't honestly say I never had fuel in it. The car was a parts pile when I got it. When I cleaned the tank with the sending unit in I could not see any signs of a previous leak

I will try it later today thanks

Also; Its common for some slop in the pressure ring and the tank fitting. Meaning it doesn't neccessarily center automatically and can in fact be installed off center. May sure you get it reasonably centered by tapping the ears in a fashion to center it. Stated another way, don't just tap one ear without regard to concentricity of the ring.

I just went through something sorta similar to this with a new fuel tank and sending unit for my '69 Charger R/T. After installing the new sending unit, then installing the tank in the car, I had a leak at the sending unit(?). Dropped the new tank, pulled the sending unit to replace the gasket with another new one, and discovered the new tank had a split approx. 3-8ths of an inch at the bottom of the hole where the sending unit installs! Was using a borrowed lift, so had no choice but to "repair" on the spot, using Permatex fuel tank sealer. Huge disappointment, considering it was a new tank, sending unit, etc., but it did stop the leak. Now I don't have any confidence in the new tank, dammit! I'll sure as hell check the next tank more closely before I install it!

Did the same thing, replaced the new bung ring and gasket with the old bung ring and gasket, and the leak stopped.

New gasket and bung ring were cheap and thin compared to the old one.

Very sad.

I had a similar problem with a new pickup. New ring, old ring, old gasket they all leaked. I found the sending unit wasn't bent correctly - the pickup was hitting the bottom of the tank and tilting the sealing flange. A little hand bending of the pickup solved the problem. Worth checking.

I fixed the leak at the bung I cleaned an reused the old gaket and ring. Now I have another leak at the tank up at the vapor lines. I used 73-74 tank with the 4 return lines on the front top of the tank. I made a new manifold because the original one was all rusty inside and out. I am using the vapor lines as fuel return lines because I am using a Holley return style fuel regulator. The instructions said that I should have the same size return as pressure line my pressure line is 3/8. My sending unit only has a 5/16 or smaller return so I ran 4 lines into the front top of the tank and used a vented gas cap. Now one of the lines are leaking at the front top of the tank. I wonder if the return lines going into the front top of the tank are causing more pressure than the vented gas cap can vent