The big end from the crankshaft travels in a cicular motion.So you might be looking at equations of a circle,or angular velocity,depending on what you want to know. Since the little end attached to the piston goes up and down ( we say reciprocates) its position (or more usually the position of the top of the piston - you might know it as top dead centre) can be expressed as a function of time ,usually something like A sin(t + omega) where t is the time in seconds.
If you are more specific about the engine speed and length of the piston etc,people on here will be more able to offer help. Should also be able to figure out the forces acting on the gudgeon pin etc. ( And yes,I do own a motorcycle which I frequently take to pieces.)
