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Flywheel key sheared off, possible causes?

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So I bought a 2004 RMZ250 last month with low compression and it wouldn't run. I replaced the piston, rings, pin, timing chain, base gasket, head gasket and shimmed the valves up. I removed the flywheel in the process so that I could change my timing chain. Put it back together, made sure the flywheel nut was tight and rolled it slowly. No interference, no issues. Hooked up spark plug boot, turned the fuel on and kicked it over. The bike fired up second kick and idled like a champ. Hauled it out to my bosses property, started it, let it warm up and adjusted the idle screw. Took it for a ride, didn't crack on it at all and only made it about 200 feet before it sputtered and died. When I was unable to get it to start it again I loaded it back on my truck and hauled it home. Pulled the spark plug boot off and connected my spark tester, kicked it over and no spark. Pulled the timing plug out of the side cover and rolled the bike by hand. The flywheel didn't roll. Pulled off the timing cover and discovered the flywheel was free spinning on the crankshaft. Once I pulled the flywheel off I found that the key had sheared off flush. There doesn't appear to be any damage to the flywheel, there's no play in the crank, what's the deal? Just one of those things?

 

Cliffnotes:

Bike lost fire, upon inspection the flywheel key had sheered off allowing the flywheel to spin freely. No apparent damage to flywheel, nut was tight, no play in the crank.

Edited by boost_junkie16

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