I don't believe anyone suggested that the tow bar be fastened to the bumper, so yes, "Attaching to the bumper is totally inadvisable" is agreed.
Attaching a tow bar base plate (home made or otherwise) to the place where the bumper attaches (and more) is the only advisable way. A plate could be made that sandwiches between the bumper shocks and the frame and another separate attachment point made, and that plate welded to the attachment points of the tow bar. I am sure (if it were my car) I could design and fabricate a base plate that would be able to lift the entire weight of the car if need be. I am also sure there are plenty of other people that could do the same or better....Rich

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All Diesel Fleet
1985 R107 300SLD TURBODIESEL
2005 E320 CDI (daily)

LOTS of parts for sale!


EGR block kit
http://www.peachparts.com/shopforum/diesel-discussion/355250-sale-egr-delete-block-off-plate-kit.html

1985 CA emissions 617 owners- You Need This!
Sanden style A/C Compressor Mounting Kit for your 616/ 617 For Sale + Install Inst.

Sanden Instalation Guide (post 11):
http://www.peachparts.com/shopforum/diesel-discussion/367883-sanden-retrofit-installation-guide.html