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gbalias

what happens when...

18 posts in this topic

the bottom end bearing fails?

ive got some slight new knocking on my 98 yz400 and im pretty sure its the crank end of the motor. so if im riding and it goes out, what other damage will occurr when it happens besides having to walk the whole way back?

:thumbsup:

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Well, it might break the bottom off the cylinder and cut a slot through your crankcases, and maybe bend all of the valves and hack up the head.

Or it might be a loose crank bolt letting the balancer drive gear rattle. Check it. If it's loose, replace the key in the balancer gear.

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the bottom end bearing fails?

ive got some slight new knocking on my 98 yz400 and im pretty sure its the crank end of the motor. so if im riding and it goes out, what other damage will occurr when it happens besides having to walk the whole way back?

:thumbsup:

Are you also seeing a drastic increase in metal flakes in your filter/filter well? The knocking (not pinging) and alarming increase in metal flakes were the precursor to the big-end of my crank going south.

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Engine goes boom. I would't ride it until I found out what the noise was. It is almost always cheaper to repair an engine that is still running than one that blows up.

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i have been noticing more metal flakes lately. thats what kinda gave me away to it.

i suppose next time i open up the oil, ill get out my magnet though cause last time i didnt do that.

and i also searched before and saw the whole thing about the balance gear...so thatll be step 1. thanks folks. ill see here in the next week or so whats goin on.

might be a good time for a big bore or something.

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The 98 YZ400 I'm rebuilding started knocking, so the previous owner stopped riding it. Good thing, it was the Crank-side rod bearing. In the short few minutes it was running with the knock, it managed to scar up the cylinder walls pretty good as well. I'd tear it down and check it out, it's a relatively cheap fix if that's what it is.

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nice.

now either i need to find a 'how-to' or borrow BACK my service manual from my friend. hehe.

thanx guys.

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so those of you that have already rebuilt your motors....

what did you go with? stock internals from a certain mfg? im looking at several options and wanted to get an idea of whats available with keeping good reliability of course.

so let the suggestions come.

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so its fine to change just the rod and bearings, as opposed to the whole crank assembly?. id like to keep the rest of it stock (compression, bore, etc) if everything else in there is ok.

i also couldnt find anything for the 400 in parts unlimited or TT store. ill keep lookin though

i guess i could go all OEM though. doesnt seem too expensive.

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It's a multi part crank, so you should probably just need the rod bearing and correlating pin. Might wanna check out the rest of the assembly as well though.

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cool....

ill definately give everything a once over. the bike has been rock solid till now. and it was even still running super crisp. just the noise was a lil concerning so id rather fix it before it gets crazy.

it might not even be the rod bearing though. i still have to get into it deeper but i miagine its time being 9 years old.

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It's a multi part crank, so you should probably just need the rod bearing and correlating pin. Might wanna check out the rest of the assembly as well though.
You can do that, but I don't recommend re-using the rod, and by the time you figure the labor to have the crank rebuilt, you don't save very much with that approach. And then there is the question of how much wear there is on the crank sprocket, which, if excessive, requires the replacement of the left half of the crank.

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thanks gray. id definitely replace the rod if i was replacing the bearing. you gotta get in there anyways. but once i do a full teardown, ill let ya know what i find.

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ya, i read up on that doing a search for "YZ400 knocking"

itd be nice if that was the problem. ill keep you posted.

thanks again

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