I've got a 2004 757 I bought new. Has always been a great machine. Only 460hrs. However after this first mowing this year the right head decided to malfunction and bend both push rods. I have the motor out and covers removed. Removed the head in question and nothing seems amiss between the valves and piston. I'm told on these the valve guides move and causes this. I haven't removed the valve springs yet to look at the guides. One thing I did notice right away is the amount of build up under those covers! I always keep it cleaned off but these covers allow a massive build up around these heads. So I guess my question is has anyone else had experience with this? Also when you put it back together did you put the shields over the heads or leave them off so you can keep the heads clean so the engine runs cooler because you can keep them clean? Any help is appreciated!