I agree. A fresh engine's parts will remain dimensionally stable for many thousands of miles. I think that a lot of what we hear about can be traced back the days of Model As and Ts where machine work and metallurgy weren't as good as they are today. Just look at the babbitt bearings that they used. Machine them tight - up to about 120lbs rotating torque to turn it over (like so tight the starter couldn't always turn a fresh motor over) and let it wear itself in to where it was happy. Obviously you wouldn't want to take one of those straight to the races.
Originally Posted by Cujet
Years ago, I worked for a company that was involved in camshaft development (and a second company that tested 2 stroke marine racing engines) . They dyno stuff was on the automotive side, I assembled engines with OEM parts and the dyno team ran them through a series of tests. Mostly for emissions and of course to meet target output. My job was to assemble the engines to a specification, time the camshafts and install on the dyno. I also disassembled the engines and measured components for wear. We took no care to break in engines. We started them up, warmed them up to 160 degrees coolant temperature and let-er-rip. Often for long periods of time at specific RPM's at full boost. The parts got hotter than they would in any normal car. So I don't believe the Porsche explanation above. The only thing that could possibly match the loading is towing a heavy load up a long hill. Not once did I see something wrong internally. Bearings, rings, cylinders and valves always looked perfect. It is hard to convey just how insignificant any wear was, and how brutal the dyno sessions were. All this talk about break in really leads me to believe that rings "seat" rapidly under load and then normal rates of wear occur. No other parts "break in".