Just to clarify....dont just use any washer.....a sealing ring is soft metal...usually copper. The washer "crushes" and fills voids as you tighten ...that is how it seals. Dont use a steel washer....won't work!

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)