I just welded in the flex pipe on my 84 300SD (126). I got a 2.5" flex piece and it was the same diameter as the pipe the flex tube butts up against. I'm not sure if your 124 is the same but I would guess that it is the same size.

Then I just cut the old flex pipe out, grind the edges clean and welded the new one in place. I attached the ends of the pipe on both sides (engine side and rear side) and just tacked the welds from flex pipe to tube. Then I took it off and welded the rest together.

A whole new section would cost at least $125 (cheapest I found online). I did it for about 1/10th of that.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD