i recently repaired a sunroof drain on mine. pretty similar issue. in my case i cleaned and opened up the drain hole, then JB-welded in a short threaded brass pipe. i did several more applications of JBW to smooth out all the edges and gaps outside and in. when the (now relatively massive) joint was smooth i put POR-15 all over it. works great!

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'77 240D, 504H, OM617.952, etc.