From my years in Ford dealer service departments the most common failure of a timing cover was corrosion. It mostly affected 3.8 V6, (Taurus/Sable and Windstar) but was seen on 4.0's as well. These timing covers had both oil and coolant passages in them and as coolant ages it becomes corrosive and Ford had more than its share of coolant system design problems that aggravated this corrosive condition. I've taken off covers to replace gaskets only to find that the edges are pitted to the point where they won't seal with a new gasket or with all the bolts out they break when you tap on them with a rubber mallet to remove them. Of course by this stage you can be so frustrated that a tap with your rubber mallet is more like a railway spike swing with a 20 lb hammer. I've never heard of a timing chain causing a hole in this engine's cover. If it were that sloppy you would definitely notice a lack in power as the timing couldn't make max advance on accel. The only timing cover holes I remember were old AMC/Jeep inline sixes. As the dist gear wore with age, it let the cam shaft 'walk' forward far enough to contact the timing cover and wear a circular hole in it. I worked with the guy at AMC who developed the 'fix'. In normal AMC 'we had no money so we bubble gum and baler twined it ourselves' fashion, he drilled and tapped a hole in the end of the camshaft to insert a threaded rod with a spring and ball bearing mounted at the end. When the ball bearing came in contact with the cover the spring pushed the cam back and the smooth surface of the ball bearing never cut a hole in the cover. And yes by the way it's true, we once fixed a Pacer in the production plant with a beer can.
