Valves break. The engine usually goes out between 400-800 hours. Pass on it. The good thing is if it goes out under warr, you get a new engine, IF the dealer is worth a crap. I would pass on the FX series altogether. Depending on the make of your mower (toro, hustler, ect) there are other engines available. Look at a Kohler Command, Briggs Vanguard BB, Evan the FH series Kawi. I highly recommend you avoid the FX. You will not get many hours from it.