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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.

pull the rings off since it is apart and slide just the piston into the bore. Make sure the head is off or the spark plug out. If you can't slide the piston into the bore with the rings off, probably the engine is junk. Don't cock the piston when you slide it because the clearance is only a few thousands. In my mind, one thing that could happen is that carbon could have been knocked loose so that it made it hard to move the piston until you took it apart. Or you have a bent rod

Resistance is probably being cause by a bent crankshaft. Hit anything will not cause the cylinder to distort.

sgkent said:

pull the rings off since it is apart and slide just the piston into the bore. Make sure the head is off or the spark plug out. If you can't slide the piston into the bore with the rings off, probably the engine is junk. Don't cock the piston when you slide it because the clearance is only a few thousands. In my mind, one thing that could happen is that carbon could have been knocked loose so that it made it hard to move the piston until you took it apart. Or you have a bent rod

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Thanks for the idea. I did what you said and the piston does slide in, so the cylinder is not as lop-sided as I thought. I therefore measured again and see that my original reading was poor work. Yes, the cylinder is a little out-of-round but within spec. However, I noticed a little chunk embedded in the wall. It looks like a stone about 1mm in size. In any case, the mystery is solved. The main problem is a bent crankshaft. It is not that the cylinder became distorted.

Rivets said:

Resistance is probably being cause by a bent crankshaft. Hit anything will not cause the cylinder to distort.

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Yes, that was also my opinion, but I had measurements showing that the cylinder was so lop-sided a piston wouldn't fit! Well, the answer is that my measurements were cock-eyed, not the cylinder. Thanks for taking the time to respond.

good job. Now you can decide about fixing it.

csigona said:

Yes, that was also my opinion, but I had measurements showing that the cylinder was so lop-sided a piston wouldn't fit! Well, the answer is that my measurements were cock-eyed, not the cylinder. Thanks for taking the time to respond.

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If you decide to repair it let me know I keep used parts in stock ~!~!.........I rebuild those Classics often ............

Let me know Mon Ami ....