The exhaust system on the '68 220D was not stainless, but it lasted until 1990 when I quit driving it. Did have a hole get blown out of the front end plate of the main muffler. It was not corroded, just got hammered by the exhaust pulses. Replaced it with some 1/4" diamond tread! Think that the soot and unburned diesel that coats the insides protects the system on the inside, which is where all the corrosion works on gasser exhaust systems. Not sure about the newer ones.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150