At the bottom of the fuel sending unit on a W126 there is a plastic ring around a slotted nut that holds the tube together. Here is a home made tool to get the nut off without stripping the fine threads off of the threaded rod that is in the middle of the nut. I used a dremel to cut a slot into a 1/8 screwdriver. The tip was already broken off of it anyway.

Attached Thumbnails

   

Odd. When I too mine apart there was a plastic piece on the nut. Basically a small piece of cut plastic tubbing. Just took it off. Then any basic adjustable (small) wrench worked. It looks like a 4 or 5 mm nut. Not sure why there would be a need to make a custom tool.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car