I was mowing leaves in an area where I normally don't mow and hit a rock sticking out of the ground. The impact kill the motor! It restarted ok but had a bad knock. Went ahead and finished grinding up leaves. Later removed plug wire, pull the starter, could very well hear the engine knock. Checked under the mower, nothing was hitting the blade, also turn blade by hand to be sure it was free. Few days later mowed the yard, motor was still knocking. Wife said she could hear it from inside the house. Was almost through and suddenly the knock got much worse, sounded like the piston was coming through the head and then the motor died. But on first pull it fired right off, the knock is gone and the engine runs ok? I don't understand it. :confused2: Someone want to explain what happen? I hit a stump with another mower few years back, also kill the motor but it wouldn't restart. Found the crankshaft pin was sheared on the flywheel. I replaced pin and motor was ok.