Look here:

http://www.peachparts.com/shopforum/showthread.php?t=36961&highlight=radiator+fix+copper

For now, I would suggest getting some QuickSteel epoxy from the automotive section of WalMart, or an auto parts store. If you do a good patch job, it will seal the leak for at least a year, but the crack will slowly continue to spread.

Use more epoxy, and cover more surface area, than you think you will need. That will help slow the spread of the crack.

In the end, water + heat will win the battle, but I have fought it successfully for 4 years with this easy, fast fix.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black