Matthias, do yourself a favor and replace the radiator ASAP, peferably before your 1,200 mile trip. A new radiator will prevent you being stranded somewhere when the temorary fix lets go. The radiator neck becomes brittle and if you glue the broken end it will fail again, likely at the most inopportune time. I replaced the radiator in our 1991 300CE after an identical experience to yours, and the repair is quite simple if you're a DIY'er ... somewhat time consuming, but straightforward and simple.

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Fred Hoelzle