On the way home from work the other night my engine died on me. It was riding along nice and smooth and all of the sudden it sounded like metal on metal scrapping and then died. I popped the clutch again to see what would happen and it was the same noise. I immediately thought piston and rings. I got home and tore the top off and the piston looks fine. The cylinder walls are clean and smooth as well. I spun the sprocket to move the piston and it moves smoothly. It still will not start and when it tries to turn over all i get is the terrible noise. Does anyone know what is wrong? I would love the help.