The piston is exposed to various forces.
Piston Force :- When the engine is running, it moves up and down constantly in th.- cylinder. At each reversing point it is braked sharply and then accelerated again.This generates mass inertia forces which act on the piston. Together with the forces generated from the gas pressure, they form the piston force.
Lateral Force or Normal Force :- This piston force is transmitted to connecting rod and the crankshaft. However, the connecting rod is only precisely vertical at the upper and lower reversing points (known as dead centre). The inclination of the connecting rod pushes the piston to the side, i.e. against the cylinder wall. The extent of this lateral force changes direction several times during an operating cycle. It is determined by the piston force and the angle of the piston crown in relation to the connecting rod axis. The lateral force can be derived from the parallelogram of forces.