that isn't an o-ring - its a hard plastic washer. When I replaced the whole assembly in January, I decided that the plastic washer was actually a dust cover that kept dust from reaching the threads of the pivot pin.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900