Freak occurance or?

I went to the track a couple weekends ago and while trying to kickstart my bike (03 YZ 450) there was an unusual amount of drag on the kickstarter. The bike started and ran for 5 seconds before dying. I thought something was wrong and pulled the spark plug. Even without the plug and no compression there was a lot of drag while kicking through. I started thinking the engine had seized. Went home and started tearing down the engine. Took off valve cover, cam caps and cams, everything looked good. Took off head and looked at cylinder walls. Perfect. Pulled off cylinder, piston was perfect. On to the clutch, nothing wrong. Took off oil filter cover and pulled oil filter out. Holy smokes, filter was full and I mean FULL of metal shavings. Pulled the side cover off and discovered the primary drive nut on the end of the crank had come loose and ground away at the inside of the side cover. Went online to order parts and started looking at the parts diagram only to discover that there is also a lock washer that goes behind the nut. Mine was GONE. I only found three small fragments of the washer with a magnet after flushing out the cases. It seems that the washer had disintegrated and allowed the nut to loosen until it hit the inside of the cover which was the cause of the drag. Luckily the only damage was to the side cover and the seal, washer and circlip where the end of the crankshaft fits into the side cover. The bike had never been apart so whoever assembled the engine must not of bent the washer tab enough to keep the nut from working loose. I didn't have a magnetic drain plug which would of told me a long time ago that something was wrong. I do now!!!

While I agree that magnetic plugs are good (I have one in each bike), it wouldn't have done much here. The shavings were 99% aluminum from the case, and they accumulated over a very short span of time, once the nut hit the case (the crank turns forward, so contact with the case would immediately unscrew the nut), so there would have been no real advance warning.

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