When I bought by 68 Dart in 2013, a new fuel tank and sender had just been installed. The tank has been sitting on a shelf for the last 8.5 years but now that the car is painted, I'm ready to put it back in. I cleaned up the outside and will be installing a new insulation pad and filler neck grommet. When removing the sending unit, I noticed that the lock ring was pretty loose fitting. We've all read posts about fuel leaks from this area and I'd like to be preemptive so I don't run into that issue myself. I've got (2) new gaskets. One was bought this past summer from Mega Parts and I just found another in a box that I bought from Year One 25 years ago. Both are the same thickness. It seems like 2 gaskets would be too thick to get the lock ring installed but I haven't tried that yet. Somewhere, I have my original Challenger lock ring and thought it might be thicker then the Spectra replacement. I looked for it last night but it's eluding me. I had another idea I thought I'd run by everyone.

I thought about adding a shim between the lock ring and the sending unit. This would essentially make the lock ring thicker. While running out the door for work this morning, I found some old rear axle shims in varying thicknesses. I put one up next to the new lock ring and it looks like it's the perfect size. Does this sound like a decent idea? I could glue the shim to the back side of the lock ring to keep it from moving around while installing.

Try and find a NOS or good used lock ring. The aftermarket ones I tried are not very good.

Hi Burdar , as stated above , Buy a N.O.S. MOPAR lock ring , or use a good used Original . They are TWICE as thick and Rugged as that thin aftermarket Ring . You WILL be taking Tank back down for LEAKs if you don't .
Ronnie

I tried the double gasket thing and it leaked worse. NOS one fixed mine. I may have a good used one somewhere around if you can`t find one.

I'll keep looking for my old Challenger lock ring. It's one of those things where I run across it a couple times a year and say "Oh, that's where that went." Then when I want it, it decides to play hide and seek. If it doesn't turn up, I thought the shim idea would let me use the new lock ring.

Owning a classic shop and having installed many tanks, we haven't had a lot of problems with leaks. if possible use the original ring, if not just pay close attention while tightening the ring down for any distortion if the ring.
If you really wasn to be 100% sure, cap off the openings and pressurize the tank with 5-10 psi of air (NO MORE). Spray any suspected areas with soapy water and it will bubble if leaking
NOTE: if using a repop sending unit you'll likely be removing it for adjustment as I have yet to find a source for accurate
Sending Units. Maybe some one else will chime in
A method I have used on empty tanks out of the car
1. install the S/U. measure the resistance of the S/U should be ~ 75-80 ohms. Flip the tank over and measure it again, Should read ~ 10 ohms
2. Another solution is
TANKS METER MATCH

Regarding the OP's sometimes-found parts:

I learned a long time ago to have boxes dedicated to each vehicle, grouped together.
Old/reuseable parts, replacement stuff, and notes/diagrams/photos in each vehicle's file.
There are few things that aggravate me worse than not knowing where things are !
(A siutation that still vexes me for some general spare stuff - I'm definitely not perfect !)

I found my old Challenger lock ring. It's soaking in EvapoRust as we speak. Not only is it thicker then the new one, but it's designed differently as well. The new lock ring sits flat, with just three small areas bent "up" to do the locking. The old lock ring looks more like a wavy washer. It's not bent...it's just designed that way. It will put a lot more pressure on the gasket once installed. The only thing that sucks is that there isn't any plating on the old lock ring. I'll have to throw a little paint on it once it comes out of the EvapoRust. Thanks

There's also the ones from DMT, stainless steel and say "like OEM".

https://www.detroitmuscletechnologi...-seal-and-ss-lock-ring-locking-retainer/

The OEM rings are definitely a tight fit !
I'd trust the DMT piece, too, based on what I've purchased there.

The only thing I'm questioning about this original ring is that it doesn't have any little raised tabs to actually "lock" it in place once it's in the right spot. It just has the three "ramps" that are smooth. I'm wondering if this one will work it's way loose.

WOW FM has something on the bay under 5 bazillion $$ LOL

I believe Frank passed on some years ago, the new owners seem more reasonable. I've recently bought some things for my 51 Plymouth from them ,reasonable proces, fast shipping, good service.

YMMV

I'm a bit leery about that eBay one from fmmpar . I haven't heard of this much needed part being reproduced and how did "more than 10" all of a sudden show up with 20 already sold. In "Item specifics" under Region of manufacture it says United States but the description has "Made in Canada". It sure looks similar to the cheap ones that come with sending units and NOS doesn't mean OEM or OEM like quality.
I still have an unopened DMT one and the thickness is obvious when compared to the supplied ones with sending units (I have a few of those too).

It's $4.00. Buy one and compare, then tell us.

I live in Canada so with the dollar exchange and their shipping my cost works out to be $25.15CAD (at the current rate when this is posted.). I might have considered your suggestion but not at that price, especially since I already have several. Thought I would just share my opinion since I've had some experience with the cheap ones vs my OEM one.

For my Duster, I got dorman 579-006. Its a tiny bit thicker than the other replacement lock rings I had.

maybe someone can measure the thickness of the new style and you can measure your older ones and we can lay this to rest?

if it's coming from Canada it's likely a Spectre part who BTW is in Canada