Given the stress of these events, one might assume the exhaust stroke would be easiest on the connecting rod. The objective is simply moving the piston to push the spent gasses through an open exhaust valve. This, in fact, is the most dangerous time in the entire combustion process for a connecting rod. As Manley’s Michael Tokarchik explains, “The reason why is there’s no cylinder pressure buffering during that cycle.” With many camshafts having at least some type of intake and exhaust valve overlap, there is no cushioning pressure to slow the piston down.

As the crank makes the turn again over and past top dead center, inertial forces continuing driving the piston on it’s upward journey. This is the end of the exhaust stroke and beginning of the intake stroke. At this point, the rod is stressed in a tensile fashion. The big end must comply with the crank and begin the journey back in the opposite direction, while the small end wants to stay with the piston and continue upward. According to Tokarchik, this is actually where Manley sees the most failures occur in connecting rods.

During all of these directional changes, both ends of the rod are stressed, which can eventually lead to ovaling the bearing bores or complete failure.