I would go to a hardware or plumbing supply store, and get some kind of metal tubing that will just fit in the hole.

I think some have used copper, but I think aluminum might be better.

I bought my behr locally from my mechanic last year. it had the reinforced neck. Check the DIY page for w124 radiator and you can see pics showing it.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U