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Will a sheared flywheel key cause a lawnmower engine not to spin?

3298 Views 8 Replies 5 Participants Last post by  de-nagorg
My riding lawnmower just broke while i was mowing leaves. I've rebuilt lawnmower engines before, and I'm fairly certain the piston rod broke, but i couldn't remember what the symptoms of a sheared flywheel are. Would a sheared flywheel key cause the engine not to rotate when you spin it by hand? It will move an inch or so and then stop like it's hitting something.

Like i said, i'm fairly certain something broke inside the engine and that it cant rotate because it is hitting the part... but wanted to check anyway to see if a flywheel key might do anything like this.

It shut down about 45 seconds after i hit a big stick or something with my blades, and i know that is what can shear a flywheel key... but it might have just been coincidence that it stopped shortly after hitting something... or that jolt might have caused the internal parts to fail.
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Thanks guys, it's probably a broken rod... was just hoping it might be the key, but after reading your replies and reading up on keys, i can see that it wouldn't really cause this condition.

stinks because i actually tore down and rebuilt this engine last winter due to a broken connecting rod. really disappointed that it only lasted one season... but i guess that's the risk you take with a rebuild. I used a new piston and connecting rod, but i reused the old journal which had just a tiny tiny nick in it... so now i wonder if that might have contributed to this failure. I'm going to tear it down just because im curious as to what broke, but i doubt i will rebuild it... probably will buy a new motor off ebay to put in it.

I debated buying a new motor last time, because i probably spent close to what it would a new engine would cost to rebuild this one... but i did learn a ton by doing so, so it wasn't a complete loss. Before i rebuilt it i couldn't have identified the carburetor, but when i was done, i was familiar with pretty much the entire engine and electrical system.

Thanks again.
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Yeah, i knew it was a risk to reuse it, but i cleaned and buffed it really good and was hoping it would squeak by since it was such a small imperfection. Also i honed out the cylinder with one of those drill honers, but it certainly wasn't 100% perfect either. Guess i now know why a lot of people were recommending to trash it instead of rebuild, haha.

Still, even though i only got 1 season out of it, i learned a tremendous amount by rebuilding it. Since then i've used that knowledge to troubleshoot and fix several different small engines that i would have previously had to send to a repair man.

Ebay has some scratch and dent 10.5HP B&S's with 2 year manufacture warranties for $340... i imagine i will probably do something like that.
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