Not sure about the 300SD. On the 300SDL, you can get the cover off without taking anything else off. There's a couple screws that hold it on. I also don't think the size is that big of a deal on the O-ring either, so long as it fits over the end of the magnet assembly and rubs on the plastic cover to add a bit of friction. The size I spec'd is what I had on hand at the time that fit and did the job!

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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]