Colin, I agree that you should positively confirm that the head gasket is the source of your oil leak. I've had the timing chain cover on our 1992 300CE resealed twice for oil leaks ... fortunately both times were under warranty. Our indeoendent advised me that the timing chain covers are tricky to seal properly. He's had better success with a different sealant than MB recommends. The name of the sealant slips my mind right now, but I'll ask him if you determine the timing chain cover is leaking.

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Fred Hoelzle