Let’s look at the connecting rod first and explain what it has to go through at high RPM’s. The connecting rod connects the piston to the crankshaft and at high RPM’s the piston gets very heavy and the piston assembly will weigh the most on the exhaust stroke. A piston assembly includes the piston, locks, pin and rings; weighs around 600 grams will actually weigh 15,000 lbs at 8500 RPM’s. This seems like a lot, but keep in mind how far the piston goes up, then it has to come back down on the intake stroke.
By the time the piston gets to the top of the cylinder all the exhaust is almost gone and the exhaust valve and intake valve are both open. So there is no cushion or pressure to help slow or stop the piston. The only thing to stop the piston at this point is the connecting rod. After the piston reached its peak point of travel, the connecting rod will pull it back down the bore. We have to also consider the weight of the connecting rod, if the rod happens to be steel you have a lot of weight pulling on the crankshaft pin. All of this together puts great strain on the connecting rod, pin and piston which in-turn all effect bearing load.