Simple question that i want to know answer to.
The 2 most general conditions are-bad main and rod bearings on the crankshaft which allows both the crank and rods to throw up past tolerance levels and the piston crashes into the valve.The other is usually a keeper pops on top of the valve and allows the valve to drop and crash into the piston,caused by high or prolonged rpm's
Simple answer, something broke.
Anything mechanical can break. Nothing lasts forever. Valves wear, bend, brake. Reasons usually from over revving, or neglect. If you race you can crack a guide, bend a valve, have valve to valve contact. Valve to piston. That's usually timing off or over revving. Mechanical timing off. Skipped a tooth or two on timing chain. Just went to George Town two weeks ago. One of the guys I ride with had valve to piston on the intakes on his CRF 450. His timing chain skipped a tooth. Luckily it was on the way back to staging.
