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Old 07-22-2012, 12:13 AM
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Default 1998 YZ125 with 2002 Cylinder

     

Hi all, I just bought a project bike a few weeks back, a 98 YZ125. I knew it needed a top end job and something wasn't right with the clutch. As I think I have the clutch issue solved I moved on to the top end, now I notice its not a 98 cylinder but a 2002. It looks as though either improper break-in or the fact that a 2002 top end doesn't fly on a 98 bottom caused some damage to the top of the piston and the ring got pinched. I have seen the cross referencing page for what year parts work on others, but also know that's not always correct info. If I can keep the 2002 cylinder I was going to run it as is most likely, with all the damage to the top of the piston the cylinder walls look almost decent, just one scar really. I hope to get some pics up tonight to get some opinions in. If the 02 cylinder is not usable as is then I suppose I will just swap back to a 98, I already have a 98 head sitting on the bench that I had already ordered. Your knowledge is greatly appreciated.

---------- Post added at 01:13 AM ---------- Previous post was at 01:00 AM ----------







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Old 07-22-2012, 06:00 AM
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looks like it was ran lean possibly or had gotten some forgin materials into the cylinder.
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Old 07-22-2012, 06:38 AM
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OK, have you checked the crank?
The piston looks like either the bearings in the crank have come apart or the tip of the spark plug plug came off and did the damage you see or mabe both. I would also look at where the ring ends are located as compaired to the cylinder transfer ports. I say this as it looks like part of the ring is missing and if it was hiting on the edge of a port it could have broken off and caused the damage too.
I am betting on the crank bearings.

Paw Paw

Last edited by PawPaw; 07-22-2012 at 06:55 AM.
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Old 07-22-2012, 07:07 AM
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Quote:
Originally Posted by PawPaw View Post
OK, have you checked the crank?
The piston looks like either the bearings in the crank have come apart or the tip of the spark plug plug came off and did the damage you see or mabe both. I would also look at where the ring ends are located as compaired to the cylinder transfer ports. I say this as it looks like part of the ring is missing and if it was hiting on the edge of a port it could have broken off and caused the damage too.
I am betting on the crank bearings.

Paw Paw
Hey Paw, quick question. When you install new rings is the rule to have the ring end gap on the opposite side of the exhaust port?
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Old 07-22-2012, 07:25 AM
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Ok, on most bike that is true, but the ring end gap will be where the manufactor places the pin in the piston for the ring to fit around. I have seen many year model engines have interchangable parts like above, but sometimes the pistons will not interchange due to the piston ring pin placement. That is why I ask him to check it to make sure. I know there were a series of Honda cylinders in the late 80's that would interchange, but the ring pin placement on the piston's were all different and some would cause the ring to catch on the cylinder transfer ports and destroy the engine if not spotted. Honda even had a major cylinder recall due to this back then.

Paw Paw
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Old 07-22-2012, 07:41 AM
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piston ring pin placement? is this where the gap lines up on the piston?

I better dive into my Service manual, I did see a part that said align ring end gap. Of course it was a black and white photo with an arrow pointing too the back side of the piston opposite the exhaust port with the piston mounted on the rod.


Last edited by FRODAD393; 07-22-2012 at 08:03 AM.
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Old 07-22-2012, 07:50 AM
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Yes, if it's a 2 stroke it will have a pin for the rings to fit around. If it's a 4 stroke there will not be a pin.

Paw Paw
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Old 07-22-2012, 08:02 AM
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I have not pulled the crank, yet. I was hoping to avoid splitting the cases. I did find some bits of the piston that chipped off when the ring got pinched (see pic 3). They were buried in the head. There are some ports that have had small pieces come off the edge (pic 1) and I have carefully ground them smooth since so as to not get hung up on the ring again. The rod seems to be tight and the needle bearings are in good shape, at least from what I can tell without pulling the crank.

Do I need to split the case and inspect the bearings?
Any thoughts on a 2002 top end on a 98 bottom?
Should I pickup a 98 top end when one comes up on ebay?

---------- Post added at 09:02 AM ---------- Previous post was at 08:56 AM ----------

Ok I see the ring pin now, the ring is in the correct position for the pin to fall in between and under the ends of the ring.
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Old 07-22-2012, 08:10 AM
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Is the ring broken off? It sure looks like it in the photo.
When you have had as much damage as this engine has had, it would be best to split the cases and check the crank. The big end bearing on the rod/crank can come apart and send parts up into the combustion chamber causing this damage. If you interchange parts, then make sure all fits correctly. The piston ring pin must not allow the ring to be over a cylinder port. The piston must not come in contact with the head. The head squash pattern must be correct for the piston.
Please don't put this back together without spliting the case and making 100% sure the crank is good or you will have bigger problems when you run it in the near furure. Gaskets, bearings and seals are cheep. Cranks, cylinders and etc. are not!

Paw Paw

Paw Paw
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Old 07-22-2012, 08:34 AM
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The ring is intact, although pinched severely on one end of it. Should I replace the crank bearings even if they look fine once I get in there? I guess for the cost of the bearings I should. Or a complete Wiseco crank kit with gaskets bearings and all for 175 seems like a good deal.
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