The filler tube on my string trimmer's Tecumseh LEV-100 is a bit undersize, I think it originally had a ring in there but that went who-knows-where a while ago.

I've been using standard o-rings in an attempt to seal it but they don't last very long.

The tube seems to be a bit too high up so the bead formed into the tube can't make a good seal. I have the tube's bracket adjusted as far down as it can go.

Funny how a possible solution comes to mind just as I am typing this, but the thought occurred that I could file the slots in the bracket longer, to allow it to come down more.

Or is there a more appropriate fix for this?

 

I found the proper seal ring on a parts mower, that goes between the tube and the plastic part on the end.
It's a soft plastic ring of square cross section, which gets compressed and expands a bit inside the oil opening when the tube is tightened down.
Seems to work ok, no leaks