let us also not forgert that this is an assembly line process- where each engine, although essentially the same thing, is different. In manufacturing there are tollerences and they can all line up on a particular engine and make it junk. Each part is different- no matter the attention to detail- (now we wont be counting Honda parts as they generally (engine) are so dang accurate that they consistently come up within a couple hundred thou on a Coordinate MM.) On the modern assembly line for the motor the motors are not even started until the cars are driven off the line- they are filled with oil after assembly and Spun a number of times and them drained and dropped in the car. that is why each car is different and some may never have an engine failure even after 10 billion racing miles and others will blow motors each week at the stop light.
I work in Engineering and Manufacturing - and we only deal with about 1-5 thou tolerance- and thats good enough for what we do- but its still hard to get everything to fit perfectly and to never have problems. we build everything by hand or CNC and its still hard to get things to match up right a lot of times. and we only do about two cars a month - I cant even think of the problems of doing 100,000 cars a month. I bet ford makes close to 200 thou duratec motors each month- so there are bound to be serious problems in at least 5%. I believe that that is about normal high standard for high output production.
