Please take theengineer's comments to heart: the filler tube does indeed need "double wrenching" to prevent twisting and damage.
Some mechanic (it was not me) failed to do that: he twisted the tube just as noted, and created a hairline crack as well. It required a replacement tube (USD $50) at the time, and then for show purposes, stripping of the black paint and re-plating in cadmium. ($100) So, use two wrenches!
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America