Ever suffer a main bearing failure?


Mar 27, 2008
A buddy of mine brought over his kids '04 KX100 saying it had locked up momentarily and was making all kinds of rude noise. We'll, i pulled the engine and opened it up and found that the right side crank bearing had disentragaded and the left side had welded itself to the crank. The counterweight had scraped against the right side case spewing aluminum shavings everywhere. It was ugly :ugg: . However, I cleaned it all up and I don't see any other case damage except the aformentioned scrape (about 1/6" deep). The jug is ok. Here's my questions:

I have never seen a main bearing fail like this. Any ideas what would cause this to happen?

Is it safe to clean it all up, replace the crank assembly, rod, piston and main bearings and go for it or should we risk a rebuild?

Thanks for the input.


dirt bike dave

Sponsoring Member
May 3, 2000
sci85 said:
Any ideas what would cause this to happen?

Well, the only time I blew up a bottom end was after about 2 miles of top gear WFO on a road.

I was worried that was hard on the motor, so I backed off the throttle and coasted without pulling the clutch in. Red hot motor + high rpm without any cooling fuel and lubricating oil = bottom end failure.

Ask the rider if they backed out of the throttle after a long straight...


Sep 7, 2004
i had a kdx 175 a few years ago that did that... It rattled for weeks as the bearing came apart and rattled around the top end. You could kick it over and you could feel it grinding and binding and.. it still ran. One day a ball-bearing hit the electrode, so it started to run wicked bad but still made it home. when taking it apart i found the exact same thing exept the top end was junk to. How is the top end? You might want to measure the case where the bearing fits in to make sure of no movement. And might want to check your main bearing bore alignment. As far as why it happened sounds like a lean condition/ air leak
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