Rod Bearings

The connecting rod does move left to right but if you have to worry about a better bearing - especially about putting a roller bearing there, there are greater problems elsewhere. Roller bearings are not little cylinders. They are little barrels (if you will). Their design is to submit to load for brief instances anywhere along their body. If these rollers were cylinders, they would require much more lubrication and they would wear out much more quickly. Their design has been very carefully studied.

We want some side to side thrust. It makes the bottom end last longer with its fog lubrication. And increasing side to side tolerance here would increase friction.