The fellow has a muffler shop and took a 2 1/8 (that's what he told me) straight exhaust pipe cut it about 5 inches longer than the now missing cat, expanded the ends most likely to about 2.4 inches and the welded the pipe in. Yes you are most likely correct that the original outside is 2.75 inches but the inside is most likely about 2.5 inches and to get the new piece into the old he expanded it to 2.4 probably and welded all the way around. He has one of those exhaust pipe expander machines that he used. He asked if it has to go thru emissions tests, and here in Onatrio no tests for diesels or for cars that are older than 1988 model year.

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1987 300 DT