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  #1  
Old 08-09-2007, 08:24 PM
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Default YZ 125 seized?

I recently installed a new top end. Went riding tonight and after a long time the rear tire froze up and the kick starter would not move.

After pushing the bike home I took her apart and the cylinder and piston are fine. However the rod will not move up and down anymore. Does anyone know what would cause this? Also, there were some metal shavings.
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Old 08-09-2007, 09:35 PM
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Default Re: YZ 125 seized?

Could you have dropped something down in the crank while doing the top end? An old piston ring or something?
It may be a lower rod bearing that went out. Tough call. If you've got shavings, something got chewed up before it froze the crank.
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Old 08-09-2007, 10:30 PM
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Default Re: YZ 125 seized?

I had something similar happen to me. It turned out to be the big end on the con rod. The shavings might be plastic and metal both from the con rod bearing. Time for a new crank probably.
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Old 08-09-2007, 10:39 PM
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Default Re: YZ 125 seized?

And while you're down there, might as well replace both of the main crank bearings.
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Old 08-10-2007, 05:15 AM
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Default Re: YZ 125 seized?

I am pretty sure I didn't drop anything. I still have the old piston and ring. Okay new crank and crank bearings sounds right to me. What would have caused this to happen? Could I have done something wrong on my piston install besides dropping something in there?

So this is what I need right? I assume I need the bearing on the right side of the page also. [Only registered and activated users can see links. ]

Last edited by mbogosia; 08-10-2007 at 05:23 AM.
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Old 08-10-2007, 07:46 AM
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Default Re: YZ 125 seized?

The Yamaha crank uses plastic inserts for counter balance on the big end connection and this plastic will melt and flow into the bearing starving it for oil. A Hot Rods rep. told me that Yamaha designs their cranks to do this in the event of over heating to prevent it from coming apart and causing a lot more damage.
I see by the rockymountainatv link that you have a 98 yz125. This year is especially prone to doing this apparently. Mine is a 98 yz125 also. You problably did nothing wrong in the re-assembly of the engine. Just running it hard can do it.
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Old 08-10-2007, 08:16 AM
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Default Re: YZ 125 seized?

Just dropped it off at the shop. He said that I probably didn't do anything wrong with the piston because the piston would have been messed up. He thinks the bearings got messed up and that in turn locked it up. The good thing is they are going to completely go through the engine while it is apart and it should be in great shape when I get it back. I am just worried about the cost of the repair.
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Old 08-10-2007, 09:46 AM
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Default Re: YZ 125 seized?

A hotrods cam will cost you less than pressing that crank apart and puting in new bearing, pin and con rod. In mine I replaced all the ball bearing races in the bottom while I was there that was about $75 worth not counting the crank bearings and seals which was $42 at rockymountainatv and the crank for $142. So that comes to $259 I spent on mine in just parts. with labor
But a lot less than another bike.
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Old 08-10-2007, 10:03 AM
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Default Re: YZ 125 seized?

I dont buy into the damage prevention story. I think someone was creative when he told it. The reason that Yamaha uses those is just to take up space. I have a 98 lower end in the cases sitting here. I also have a crank from one sitting here. The crank will balance "BETTER" without the inserts, which are there for increasing primary compression. The rod side of your crank is quite a bit heavier than the blank side. Find some straight edges and set it up on them and see if I am not right.

For years people have been "dremeling" that plastic away to get better oil access to the lower end bearing.

I doubt you did anything wrong either, other than like many of us just kept the crank too long. With a new top end developing power more like it was when new, it just sped the failure process up a little bit.
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Old 08-10-2007, 11:17 AM
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Default Re: YZ 125 seized?

Quote:
Originally Posted by AV8R View Post
The Yamaha crank uses plastic inserts for counter balance on the big end connection and this plastic will melt and flow into the bearing starving it for oil. A Hot Rods rep. told me that Yamaha designs their cranks to do this in the event of over heating to prevent it from coming apart and causing a lot more damage.
.
I'm going with Ossa on this one. The plastic is there to reduce crankcase volume and is used to keep the crank lighter than if their area were replaced with steel.
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Old 08-10-2007, 12:19 PM
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Default Re: YZ 125 seized?

AV8R, you sure have some misleading info.
"A hotrods crank will cost you less than pressing that crank apart and putting in new bearing, pin and con rod" Not true!

"The Yamaha crank uses plastic inserts for counter balance" Absolutely not true!

"This year is especially prone to doing this apparently." Where do you get your information????

It is fact...cranks will eventually fail in time. This being a '98, has the crank evern been replaced or rebuilt?? If not it has been on borrowed time for many years! Tdub
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Old 08-10-2007, 01:09 PM
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Default Re: YZ 125 seized?

"If not, it has been on borrowed time,,,,"
My son is cleaning out my garage. He found two YZ cranks, 3 for a cr 250 of the 92-2001 variety, and two for a CR500. He said "Dad, they all seem good." I know, it is why I keep them. All used and chances of me using them again are slim to none (except for the CR500 cranks). They got replaced because "it was time". In the case of the ones for the XR500, XL350, XR600, XL600, XR650L and all the british and spanish stuff are things I will use again. Probably not the XR250 stuff though. Anyone need any?
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Old 08-10-2007, 01:15 PM
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Default Re: YZ 125 seized?

I'll trade you a used YZ front fender, a cut down YZ seat and an old pickle jar full of misc bolts, nuts and screws for one of the YZ cranks.
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  #14  
Old 08-10-2007, 01:29 PM
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Default Re: YZ 125 seized?

I could use the jar I supose.
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  #15  
Old 08-10-2007, 02:04 PM
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Default Re: YZ 125 seized?

Quote:
Originally Posted by ossagp1 View Post
I could use the jar I supose.
Okay. Do you the misc. junk that's in it, too? If not I'll toss the contents out to save on shipping.
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