iston Rings & Piston Pins.
A Piston ring does not rotate much usually. When assembling piston rings the gaps should be staggered as per the manufactures recommendations. Upon careful disassembly of a Suzuki cylinder block you will notice that the piston ring gaps are usually fairly close to where you placed them. The ring gap stagger of properly fitting rings is why you do not see streaks in the cylinder walls. The lack of streaks in the cylinder walls is reason to be joyful and not a reason to seek out why there is no streak.
Piston pins do rotate, this is because the actions during the different cycles exerts rotating forces which tends to rotate the pin. The pin should be a slide fit in both the piston and the rod allowing and causing oil to work between all contact areas.
Under rotation the piston pin tends to wear in a even pattern. The speed of rotation is not a consequence, it is a product of the RPM, friction, viscosity, and stroke forces. Rotation patterns on the used piston pin is the results of a piston pin fits correctly and allows oil to get to all contact areas. When a pin does not show any evidence of a rotation pattern in a disassembled engine that did not have a piston, piston pin or connecting rod failure it is of no consequence, since the engine is now disassembled and already did not have a piston, piston pin or connecting rod failure.
When reassembling the engine, be sure the pin rotates freely inside the rod and the piston. If a piston pin fails to rotate it is rarely the cause of a failure, something else will ususally fail many hours before a stuck wrist pin. If a stiff pin fails it is because of major lubrication problems causing the pin to seize in the connecting rod and the piston.
Wrist pin rotation also assures that the connecting rod can seek and maintain the best spot side to side between the piston’s bosses, which should be perpendicular to the wrist pin and crankshaft. With excessive rod journal bearing clearances at the crankshaft thrust bearings the connecting rod will often tend to favor one end of the journal and become less perpendicular to the crankshaft and piston pin.
This causes the connecting rod to move diagonally towards one of the piston bosses and causes the pin to be pushed against one of the piston pin keepers. This may cause the pin to stick in the connecting rod or the piston bosses. This is not caused by a failure of the piston pin but is a result of a connecting rod journal bearing and thrust bearing being worn out. An engine with this condition is in serious need of a complete rebuild and would soon fail due to a journal failure bending a connecting rod and stuffing it out the side of the engine block.
Proper lubrication is the key. If you are investigating wrist pin rotation and compression rings not leaving streaks you are wasting your time, you should be more concerned with the fit of the journals and thrust bearings because these are where upper connecting rod and piston skirt problems tend to begin