OK, now we are getting into semantics. The roller bearings allowed for the higher rotations capable with the shorter stroke. Improvements in heads and the overall metallurgy also played a role in all this, but the point is that the advancement of 4$ technology to produce more horsepower came at a price and that price is high revs. The tax placed upon that cost of higher revs, regardless of engine design, is reduced reliability.
Back in the day, the 4$ was dead reliable because it was long stroke (compared to a 2T) and turned slowly. Today, the tables are turned with the 4$ having a short stroke and revving to the moon and the 2T being considered reliable precisely because of the lower revs per HP produced.
@2T Institute - Disagree with you completely. DI may not be the answer of all our ills but we will see it and soon. It is already available in outboards and snowmobiles. It is only a matter of time. And the reason may surprise you. They will not produce it for America though we may see it here. It will be produced for the developing markets (India, China, Brazil) where the 2T is more common and emissions more of a concern.