A friend brought over his dead little mower. It has a 3.5hp Briggs & Stratton engine of relatively recent vintage (2012). He hit a rock and the engine now can barely be rotated. You have to grab the blade and turn hard. He has already bought himself another mower, so this question is merely a matter of investigation, a sort of post mortem.
What I see is that the resistance is caused not so much by a bent crankshaft as by the piston. The flywheel key did shear, and moved about the width of the key. I presume the crankshaft is bent, but it's not obvious to the eye. With the connecting rod disconnected from the crankshaft, there is still a little resistance to turning the crankshaft, but much, much less so. The largest share of resistance comes from the piston no longer fitting the cylinder. I pulled out the piston and measured the bore. It's significantly out-of-round, about 0.5mm (in a 65 mm bore) difference between a measurement in one direction and another at 90 degrees. The piston skirt is scratched but only a little. The cylinder itself shows just a little scoring but almost no wear. Rings are good, not scored. And so my question...
Is it possible that hitting the rock shocked the engine body so much that the cylinder deformed? Has any of you experienced something like this? It's a first for me.
What I see is that the resistance is caused not so much by a bent crankshaft as by the piston. The flywheel key did shear, and moved about the width of the key. I presume the crankshaft is bent, but it's not obvious to the eye. With the connecting rod disconnected from the crankshaft, there is still a little resistance to turning the crankshaft, but much, much less so. The largest share of resistance comes from the piston no longer fitting the cylinder. I pulled out the piston and measured the bore. It's significantly out-of-round, about 0.5mm (in a 65 mm bore) difference between a measurement in one direction and another at 90 degrees. The piston skirt is scratched but only a little. The cylinder itself shows just a little scoring but almost no wear. Rings are good, not scored. And so my question...
Is it possible that hitting the rock shocked the engine body so much that the cylinder deformed? Has any of you experienced something like this? It's a first for me.