Piston Pin
A piston pin transfers the entire power out of the combustion process from the piston, via con. rod,

to the crankshaft and is the most stressed part of an engine. 

Bearing
It helps to reduce friction and to ensure that the motive power of the engine is reliably transmitted to the wheels. The functions demanded of motorcycle bearings include facilitating a smooth ride due to low friction and vibration, and ensuring durability.