Finally bought a used Exmark 60" ZTR with 1275 hrs and a near-new 27 hp Kohler repacement engine....when I picked it up it sounded ok and worked fine. Brought it home where it sat on the trailer for a few days until I had time to mess with it. As soon as I started it up to get it off the trailer the engine started knocking loudly and only hitting on one cylinder and backfiring badly....was just able to get it up to the shop. I was cussing the guy that sold it, thinking he'd found some way of hiding a big problem. Started checking it out: - had good spark - had good compression on both cylinders - timing was right - not a fuel problem since right cylinder was firing fine Finally pulled the left valve cover and YIKES!...intake pushrod bent and exhaust rocker bolt hole stripped out. What the heck? Went ahead and pulled the head and found the problem....there was a little nugget of steel sitting on top the piston. (see pic) It had jammed against the valves hard enough to do the pushrod/rocker damage....put little dings in the head and piston but not enough to hurt. The cylinders look great...no damage and no wear so he was telling the truth about it being a replacement engine. The hydraulic lifters look brand new too...could hardly see where the cam had touched them. Only thing I can think of is that something shook loose bouncing down the road and it sucked it in as soon as I started it. Well, I just straighted out the pushrod, heli-coiled the stripped bolt hole, touched up the valves and seats with lapping compound and put it back together. Started right up and runs great. Can't tell what the piece of steel was and couldn't find where it came from. Other than changing the oil and filter the repair didn't cost me a nickel.