I splurged and bought a genuine mercedes part (seal ring N 007603014100) for a whopping $1 or so. And it actually worked (for my 68 220D). No more leak for me. The engine was black from years of this leak occurring, so I will be interested if it holds or starts leaking again soon. The seal ring had a more rounded cross section than the copper washer it replaced. I think the rounded shape helps it deform a little to help the seal. So a flat copper washer probably needs to be replaced. I also scrubbed the area with fine sandpaper to clear off 45 years of gunk.

As a potential backup, I called two hydraulic equipment stores near me in central CA, and neither had heard of Nu-Lip washers. I described it as best I could, and I think they have similar items. But the name "Nu-Lip" is not commonly used around here.

Thanks for the part number. I've ordered a handful of N 007603014100.

I'd been using what the PO had included with the sale. They look flat and I doubt they're the correct part.

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1974 240D "Boldie" 170K.- New timing chain/freshly rebuilt IP/replaced valve seals/injectors/upgraded stereo/new Bilsteins with Yokohamas/fresh paint and rocker panels plus lots of welds.