OK, just to muddy the waters a bit- - - -the most dangerous point in a 4-stroke cycle is the top of the exhaust stroke. The piston and rod assembly is heading up the cylinder with virtually no opposition. The exhaust valve is open, the exhaust charge is leaving the cylinder, and if the pipes are designed right there's a little vacuum being developed to help scavenge the cylinder. Centrifugal force on the connecting rod and rod bearing is maxed out, and as the speed is increased, the rod can actually stretch. No such problem on compression, as the gases in the combustion chamber are exerting a push force on the rod and bearing.
Add a somewhat long-duration camshaft, and the intake valve will be in the process of opening toward the upcoming, no-load piston at the top of the exhaust stroke. A little high-RPM valve float- - - -a little rod stretch, a slightly loose rod bearing- - - -crash time!
Jerry
