Crank-pin bearing out of round.
Main bearings of crank shaft loose.
Bearings too tight.
Wrist pin loose in piston.
Wrist pin loose in upper end of connecting rod.
Wrist pin not in line with crank shaft, causing side slap.
Piston ring loose in slot or broken.
Cam follower guide won.
Cam loose on shaft.
Worn cams (Flat spot).