Ed,


In a two-stroke engine, firing normally and constantly on every revolution, the connecting rod is always under compression load.
This load does alternate.
It is stronger as the piston is closer to TDC (by compression and combustion forces from above the piston and from inertia of the crankshaft and prop-load from below) and weaker as the piston is near BDC (prop-load and crankshaft inertia 'try' to slow the piston as it descends and then crankshaft inertia pushes it back up).
At mid-stroke; both on the way up and on the way down, the forces on the con-rod are virtually zero.

But they are never at a real negative value.

I.e. a two-stroke engine running normally, is never 'trying' to pull its con-rod apart...


In a four-stroke engine it is different. At nearly all phases of operation, it is very much like a two-cycle engine and the rod is under compression, but at the end of the exhaust phase and the beginning of the intake phase, there is virtually no force applied to the piston from above.
...And this coincides with the highest piston acceleration (deceleration, actually) rate... The piston 'wants' to continue going up, but is pulled down by the con-rod. The forces applied to the rod are of tension and as RPM is increased, these forces are too, at an exponential rate.

So, a four-stroke engine 'naturally' applies tension forces to its con-rod, whereas a two-stroke never does, unless it is very tight (to the point the piston catches at TDC) and is broken-in at a cold four-cycle...


...And, not all four-stroke engines are designed for lower RPM...