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Joey Obrien

Check 400ex crank bearings

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I am about to get a seized 400ex, but first I have to inspect everything else. To check the crank bearings, can I just pull the flywheel cover and wiggle the flywheel and look for play there? Also, what else should I inspect for? The reason it seized was he put on an hmf but didn’t jet it so it burnt up. Any help is appreciated, Thanks

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Unlike a 2-stroke, too lean jetting will not typically seize a 4-stroke,

it will run hot, risk detonation, erode the piston crown but likely end up

burning a hole in the piston / loose compression or burn an exhaust valve before any seizure would occur.


If the engine suffered a major failure, it wouldn't be wise to only partially rebuild it (say just the top end)

without fully dis-assembling it top-to-bottom to closely inspect everything.

The flywheel 'wiggle test' might show major slop in the bearing but won't tell how smoothly they spin,

another consideration is failure debris lodged deep inside the cases that will eventually make itself loose and create havoc.


No one ever wants to tear into engine cases but some prefer to learn the hard way,

wrecking their newly rebuilt top ends when the engine soon throws a connecting rod on their first ride.


Very first thing you need before getting started is the factory service manual specific to this model/year.

Edited by mlatour

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