I did a valve job today on my 1980 300TD. The job went perfectly until I put the valve cover back on with the new gasket. No matter how hard I tried to tighten it, the oil woud just seep around the gasket. I tightened it more and more till the bolt snapped. How do I get the bolt out and replace it? Why would the new gasket leak like that. I don't understand the HARD plastic gasket and why Mercedes uses ONLY 4 bolts????