There are tons of things that go into making an engine reliable. One of the major factors is the fit of the parts, and the surface finish of the moving parts.
For instance. Let's say I'm cutting a punch for a transmission spring. There is about .01ish tolerance on the fit, of the die and the punch. Now, I can hold that all day long on my lathe, but because of how the parts interact it would seize almost instantly.
Same idea happens for things like bearing surfaces, cylinder bore and piston. A piston is sized so that when the engine reaches operating temperature the cylinders seal properly. When you changed to forged pistons the expansion rate changes compared to what was engineered to begin with. This means piston to wall clearances are much more open allowing piston slap and blowby until the engine is at temperature, and or under heavy load. You also have a tolerance for the perpedicuarity of the cylinder bores. Many companied bore them from the deck, when in reality they need to be bored perpendicular to the bearing surfaces for the crank so the pistons are moving in as close to a straight line as possible. If your pistons are moving laterally there will be excessive wear on the cylinders and that causes loss of compression and a dead block.
The cylinder wall finish is also important and takes more than. Just hitting it with a hone to look scored. There is an surface finish call out to ensure that it's smooth enough to seal against the ring, but has enough to wear in the ring to the shape of the bore.
I've been spending much time on the topic of building my car. There is much information on this as why built engines don't survive as long when built. Several of our own members here can attest to the shortened life.
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