I had one put in last year into a 1989 420SEL. The mechanic put it in three times before it stopped leaking. He finally used one of those thread sealers, good to 3000 PSI and petroleum products, (available at home depot). The tape did not work on it, he tried twice. If you don't do it right the first time, you'll be draining the coolant on each subsequent try, so may want a little extra on hand.

He used a cold chisel to start the plug turning, when taking the plug out.

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1989 420 SEL