The plastic tanks use an o ring type seal and are crimped on using a flange that is part of the aluminum core.

I've tried twice to effect a repair by swapping tanks, and both were unsat. Local radiator shop doesn't recommend this repair any more either.

When you recrimp, the aluminum seems to lose it's ability to hold the seal, and the failure mode is a weakening of the crimp, leading to leakage and/or a blowout of the seal.

Just get a new one, or if the cost approaches the car's value, find an original untampered used one.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles