I have a 98 yz400. The left side intake valve has stripped the cotters on multiple occasions. I can't be sure if the valve is sticking and causing this or if the valve just becomes stuck as a result of being struck by the piston. each time this happens I replace the valve and inspect the cylinder head but can't find any visible defects. Bottom line......stripped cotters, stuck valve, valve bent from piston. Not sure in what order. Please help, this is driving me crazy.