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When the engine is running, the piston moves up and down in the cylinder. When the piston reaches the turning point, it slows down and then accelerates again suddenly. This produces inertia forces that act on the piston. When considered together with the forces generated by the gas pressure, this forms the piston force, which is transferred to the connecting rod and crankshaft. Connecting rods are only perfectly vertical at the upper and lower turning points.
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