the 8mm x 2.0mm O-ring that is usually recommended and shown in writeups did not work for me, too small OD and still leaked. I used a 7mm x 2.5mm and worked perfectly. The thickness is more important than the diameter, you can always stretch a smaller O-ring slightly.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]