My gf ran the oil too low and it caused the timing chain to jump a tooth. We messed around with the dist. timing and it runs ok now. After reading how rediculous it is to replace the timing chains etc on the altima, I think I am just going to let it like this and just drive it. The car has 150k on it. The design with the timing covers sharing the head gasket as a seal it redic. You have to pull the head to replace the timing set.