It is very easy to roll the U shaped seal when reinstalling the upper chain cover. A rolled seal will leak. The lower timing chain cover does not need to be removed when replacing a headgasket, so it is very unlikely that the front crankshaft seal was affected.

J. M. van Swaay

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1994 E320 Wagon, 230,000 miles
1995 E320 Sedan, 106,000 miles
1994 E500 Sedan, 79,000 miles