So I was riding down the trail a week ago on my 2000 yz426, and a valve broke and lodged into the piston and wreaked havoc on my head. Does anybody know how or why this happened or what I can do so that this doesn't happen again. Heads are kind of spendy (know where to find a good used one?). I had the valves checked about 2 years ago and the shop said they were fine. I don't ride all that much. Any help?