Changing an Engine Timing Chain        

4 Stroke engines have been running overhead cams almost exclusively for decades now; they reduce friction and valvetrain weight compared with the old pushrods, making for higher revs, more reliably. And the key to keeping it all from smashing into one another is the chain that links the crank and the cams. It's cheap insurance to keep it in check & adjusted.
Courtesy of Transmoto Magazine.