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Hey everyone. I thought I knew what I was doing with this engine right up until I didn’t.  Now I’m doubting myself.

My son’s 2001 KX125 died the other day. The engine wasn’t seized so we pulled the cylinder and found it was scored quite badly. Unexpectedly, we found a sleeve in the cylinder that had been bored to 56mm. I don’t like sleeves but financially it makes the most sense to replace it as a new cylinder is $700 and only available through Kawasaki.

so here’s my question...what size bore can I use with a new sleeve? I believe I have to go back to 56mm bore based on this reasoning. When the previous owner bored this engine they must also have put on a bigger head and trimmed the power valve right? So if I wanted to go back to factory spec of 54mm I would need a new head and power valve? Please tell me if I’m wrong.

 I hate to govstraight to an oversized piston but I feel like it’s the most financially feasible repair. He doesn’t race at all. It’s just a “for fun” bike.

thanks in advance for any help

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Kenoconnorracing.com

tons of information within his site

he has a phone number posted on it too. 860-230-5003

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Ken is my go to guy. Just call him and you will know your in good hands. Great guy great service. He does everything on my 98 KX 250. 

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Thanks guys. I appreciate the info. I’ll post back with whatever I find. I think my logic follows but it’ll be interesting to see what somebody who knows what they’re talking about thinks.

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The sleeve is quite possibly the reason it scored the piston in the first place.  Poor heat transfer characteristics, hot spots are likely and they result in scuffing and seizure.  I would put that jug out to pasture.

You could order a good used cylinder:

https://www.ebay.com/itm/1999-99-KX125-KX-125-Engine-Cylinder-Jug-Kawasaki/283948220094?hash=item421ca1febe%3Ag%3AOMcAAOSwQClfDfbl&LH_ItemCondition=4

And a power valve:

https://www.ebay.com/itm/99-1999-Kawasaki-KX-125-KX125-Cylinder-Power-Exhaust-Valve/283946853901?hash=item421c8d260d:g:lNEAAOSwf9pfDLa0

Usually there are more options on used cylinders, these are all I see right now.  Ideally you might find one for $50 that needs a replate, might even come with a power valve.  Eventually one will pop up, could be a while though.  Whatever you do, don't bother with a sleeve.  They are less durable, less reliable and unless they do a top tier job matching the porting it will never run properly.

 

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I share your opinion about the sleeve, i just didn’t want to spend $1500 fixing a $1500 bike if I don’t have to.
 

Since you brought up the subject of an ebay cylinder I’ll ask this. I haven’t been able to find a straight forward answer as to what the difference is between different years of cylinders. Part numbers show that only a 2001 cylinder will work on a 2001 engine but nobody can tell me why. Most ppl guess that it has something to do with the power valve. Can I put a ‘99 cylinder (for example) on his 2001?

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If stock bore is 54mm and you are at 56mm then those valves would definitely have to have been trimmed about a mil at some point which is not hard to tell just by looking at the tips of them. To put it back to 54mm you will need new factory length valves and an OEM sized head gasket, I have never heard of anyone altering the head for a 2mm overbore. 

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Also check your piston to powervalve clearence on the setup you have now with feeler gauges and let us know what you get, if it's anything over .015 then they have most likely been trimmed. I'm in the process of building a 1990 rm 257 engine right now ( that's 2 mill over) and I had to relive the valves roughly 1 mm. If I were you I would find someone who does a cylinder core exchange online and get yourself a new replated factory bored cylinder and good used valves on eBay because the new ones they want an arm and a leg for, check your piston to cylinder clearence with spec before installing so you know it's for the correct bore aswell as your piston to powervalve clearence which is at minimum 0.015 or you will start running into clearence issues with the piston.

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1 hour ago, Sbcwrangler said:

Since you brought up the subject of an ebay cylinder I’ll ask this. I haven’t been able to find a straight forward answer as to what the difference is between different years of cylinders. Part numbers show that only a 2001 cylinder will work on a 2001 engine but nobody can tell me why. Most ppl guess that it has something to do with the power valve. Can I put a ‘99 cylinder (for example) on his 2001?

Porting changed year to year so the part numbers are different.  You can cross reference base gaskets, head gaskets and power valve parts and you'll see more interchangeability. 

98-02 will all bolt on and run.  99-01 use the same power valve, which was an improved version of the 98 valve.  They improved it again in 2002, the last year was the best design.  The cases were redesigned somewhat in 99 or 2000 so the transfer ports are a little different at the base gasket.  You could run an early cylinder on a later motor, I doubt you would be down on power, but the rear transfer tunnel is larger on the later engine.

When I did it I went the other way, I ran a 2002 cylinder on a set of 97 cases.  I just ported the cases to match the cylinder.  In your situation you could hold out for a 00-02 cylinder and matching power valve parts or you could run the 99.  With or without matching the cylinder to the cases, I'm sure it would run strong.

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