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There is no set of rules. In most motors, as they accumulate miles over years, the two main modes of failure from wear will be rod bearings or valves / seats.The cause of rod knock is that there is too much of a gap between the bearing and journal. The knock can be at random times and rpm's due to variations in temperature, oil pressure, oil flow, harmonics from the piston moving in the cylinder and other factors of course. You can have a rod knock for a long time without ever actually spinning a bearing which is the ultimate failure, ie: I had a ford truck that developed a cold-start rod knock at around 100k miles. It was still doing it when I sold the truck 10 years later with 270k miles, and it ran just fine. A rod bearing spins after there is so much wear, that the oil film between the bearing and the journal can't prevent the bearing from physically contacting the journal. This is when enough heat is generated that the bearing material actually melts into the crank material. In the whole 6 years that I've been involved with Z cars, I've personally observed 5 running motors with rod bearing failure, This leads me to believe that the VG motor is more prone to bearing failure than ring/piston/valve/HG failure.
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07:54:19 09/17/10
