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Kodie

YZ125 How to fix scarred up yz125 cylinder & piston

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I have an 03 yz125 and I’m pretty new to dirt bikes but the guy I had bought it from didn’t tell me certain things and so long story short i was running it wide open and just lost all compression and tried to start it back up and I knew something wasn’t right so I took my cylinder off and everything was all scarred up and my piston looked like someone shot a firework at it. It’s an A size piston and I think stock bore is 54x54.5mm. I was just wondering what I need to do next. I want to try and avoid getting a whole new cylinder plus the top end kit. I was thinking if it would be just as effective to bore it out to the next size up and get a piston kit to fit. 38A86458-27B5-4D9D-8DDF-B9FD86EB3187.thumb.png.615435955b0facd3cd7ee7c3c188895f.pngACA4CC24-577F-48CE-B339-529C62E6390C.thumb.png.2bc71b99e3185e7ac85bf351ddd52b4f.pngD0BE0F51-9F91-45A5-B9A1-C4E0D0E236D5.thumb.jpeg.57fe3d12f79a2ddbd8a2ca644de9c31e.jpeg

Thanks, Kodie 

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Ugh that sucks! Need to determine why it happened. No photos provided of cylinder however it's probably scraped up too. You can likely buy a new piston and send the cylinder and new piston to Millenium Technologies for replating and repairs. They do good work and have sent my own stuff there before. Probably run you about $200 for the replating + shipping and taxes and whatever a piston goes for. Be sure to get new gaskets too.

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Kodie, also inspect the connecting rod/crank for any up/down play.

You'll also want to confirm the engine doesn't have any air leaks before running the engine once rebuilt,

ideally finding the cause of the damage (wrong jetting, lack of oil, air leak etc.) to avoid repeating it.

Edited by mlatour
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Looks like an owner issue. New -ish piston. Run WOT for too long, dry seize. You have a MX bike, designed for track riding, not flat out on long straights. You must change oil ratio, jetting and other things if you are just going to rip down the road. If you do not, this will happen again.

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12 minutes ago, mlatour said:

Kodie, also inspect the connecting rod/crank for any up/down play.

You'll also want to confirm the engine doesn't have any air leaks before running the engine once rebuilt,

ideally finding the cause of the damage (wrong jetting, lack of oil, air leak etc.) to avoid repeating it.

Pretty sure my oil ratio was incorrect too

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42 minutes ago, catmanz440 said:

Ugh that sucks! Need to determine why it happened. No photos provided of cylinder however it's probably scraped up too. You can likely buy a new piston and send the cylinder and new piston to Millenium Technologies for replating and repairs. They do good work and have sent my own stuff there before. Probably run you about $200 for the replating + shipping and taxes and whatever a piston goes for. Be sure to get new gaskets too.

The cylinder looks just as bad as the piston

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BUMMER: Unfortunately you have operator error. 

Jug will need to be replated, new piston will need to be matched to the bore. Make sure the PV didn't get damaged. 

Disassemble the cylinder by removing studs, and power valve components, box it up and send it with a new A piston, or purchase from replate shop. My shop uses PowerSealUSA for this type of work. 

You will need to inspect your bottom end and make sure it didn't suffer damage. 

Clean the carb, mark down your jetting for reference, and come back on and let us know what you have.

Note:

Sell the bike if your idea of riding an MX bike is pinned WOT, there are other bikes better suited for that. 

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1 hour ago, YZDOC said:

BUMMER: Unfortunately you have operator error. 

Jug will need to be replated, new piston will need to be matched to the bore. Make sure the PV didn't get damaged. 

Disassemble the cylinder by removing studs, and power valve components, box it up and send it with a new A piston, or purchase from replate shop. My shop uses PowerSealUSA for this type of work. 

You will need to inspect your bottom end and make sure it didn't suffer damage. 

Clean the carb, mark down your jetting for reference, and come back on and let us know what you have.

Note:

Sell the bike if your idea of riding an MX bike is pinned WOT, there are other bikes better suited for that. 

I did not intend to have it wide open for as long as I did. I was getting from a dirt road to our track in the woods and didn’t have the knowledge of what was going to happen. It would be better replated than to be bored out? Thank you for the help. 

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1 minute ago, Kodie said:

I did not intend to have it wide open for as long as I did. I was getting from a dirt road to our track in the woods and didn’t have the knowledge of what was going to happen. It would be better replated than to be bored out? Thank you for the help. 

You really do not bore a nickel plated cylinder unless the scuffs from piston are minimal and you are just cleaning up the crosshatching (you need to know what you are doing). There are only 2-3 ways to go about it, and that is to have it replated, bored and steel sleeved, or a new OEM jug. Highly recommend the first as steel sleeves are last resorts, and new may be $$$. Replate: count on $199.00 to hone/bore/replate, and if any welding is needed add $60.00, now add the shipping to and from and you are around $300.00 w/o a piston. Pistons can be $54.00 up. Do not buy a cheap Namura piston kit. Recommend a OEM, Vertex, ProX, Wiseco, Athena, Wossner, and NEVER, EVER Namura. 

So, look into what a new OEM jug would cost and add piston, you will probably save a little money on the replate. 

PowerSealUSA has a work order PDF and you can fill that out if you send it in. Pistons can be purchased through them, but you might save a little more if you order one (A size) and ship it with the jug. $13.85 is what a flat rate USPS box cost, but they will ship it back UPS/FedEx and will be poss $30.00. 

Good luck, and ask away, we are all here to help. So, check everything else out so it doesn't happen again. 

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1 hour ago, YZDOC said:

You really do not bore a nickel plated cylinder unless the scuffs from piston are minimal and you are just cleaning up the crosshatching (you need to know what you are doing). There are only 2-3 ways to go about it, and that is to have it replated, bored and steel sleeved, or a new OEM jug. Highly recommend the first as steel sleeves are last resorts, and new may be $$$. Replate: count on $199.00 to hone/bore/replate, and if any welding is needed add $60.00, now add the shipping to and from and you are around $300.00 w/o a piston. Pistons can be $54.00 up. Do not buy a cheap Namura piston kit. Recommend a OEM, Vertex, ProX, Wiseco, Athena, Wossner, and NEVER, EVER Namura. 

So, look into what a new OEM jug would cost and add piston, you will probably save a little money on the replate. 

PowerSealUSA has a work order PDF and you can fill that out if you send it in. Pistons can be purchased through them, but you might save a little more if you order one (A size) and ship it with the jug. $13.85 is what a flat rate USPS box cost, but they will ship it back UPS/FedEx and will be poss $30.00. 

Good luck, and ask away, we are all here to help. So, check everything else out so it doesn't happen again. 

Almost positive the replating is what I’ll do. I wouldn’t have known about powerseal if you hadn’t mentioned it, I’m learning this bike by all the things I’ve had to rebuild or replace, and here of course. Hopefully the bottom end isn’t destroyed but I guess just pushing down the Kickstarter isn’t enough to tell if the trans and other components are still intact. I’ll let y’all know when things are done, fingers crossed 

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4 hours ago, Kodie said:

Pretty sure my oil ratio was incorrect too

That will do it ! Run 25ml of oil to a litre of fuel to be safe

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5 minutes ago, KTM TRAGIC said:

That will do it ! Run 25ml of oil to a litre of fuel to be safe

Run 33ml of oil to a liter of fuel to be safe. 40:1 is good for woods bikes, not Mx bike running wot through most of the gears. 

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3 hours ago, Kodie said:

I did not intend to have it wide open for as long as I did. I was getting from a dirt road to our track in the woods and didn’t have the knowledge of what was going to happen. It would be better replated than to be bored out? Thank you for the help. 

If you wanted to bore it out, now would be a good time to do it.

To repair what you have they would bore it out just slightly and then plate it back to oem specs.

If you wanted a bigger bore they would bore it out to whatever size you choose then plate it. Then they would modify your power valve, porting and head to work with the bored cylinder. Since you’re already paying for replating, to go to a bigger bore you would only be adding the cost of modifying the power valve / porting - probably ~200 extra to go big bore vs rebuild it stock. Not saying you need a big bore or anything, the stock 125s are tons of fun, just letting you know the options.

If your cylinder is gouged pretty deep you might be forced to bore it out, it doesn’t appear to be that way but the cylinder plater will make the final call.

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11 minutes ago, OKDinosaur said:

If you wanted to bore it out, now would be a good time to do it.

To repair what you have they would bore it out just slightly and then plate it back to oem specs.

If you wanted a bigger bore they would bore it out to whatever size you choose then plate it. Then they would modify your power valve, porting and head to work with the bored cylinder. Since you’re already paying for replating, to go to a bigger bore you would only be adding the cost of modifying the power valve / porting - probably ~200 extra to go big bore vs rebuild it stock. Not saying you need a big bore or anything, the stock 125s are tons of fun, just letting you know the options.

If your cylinder is gouged pretty deep you might be forced to bore it out, it doesn’t appear to be that way but the cylinder plater will make the final call.

Good option but when the dust settles you might be only a few dollars away from a complete Athena BB kit. Then you would also have the original to go back to if need be. 

https://www.ebay.com/itm/Athena-Comp-Big-Bore-Cylinder-Kit-4mm-58mm-144cc-14-0-1-Yamaha-YZ125-1997-2004/333407543779?fits=Model%3AYZ125&epid=171300640&hash=item4da0a361e3:g:fi4AAOSwt-Bd12a-

Eh, maybe not... 

A few dollars

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I have an 03 yz125 and I’m pretty new to dirt bikes but the guy I had bought it from didn’t tell me certain things and so long story short i was running it wide open and just lost all compression and tried to start it back up and I knew something wasn’t right so I took my cylinder off and everything was all scarred up and my piston looked like someone shot a firework at it. It’s an A size piston and I think stock bore is 54x54.5mm. I was just wondering what I need to do next. I want to try and avoid getting a whole new cylinder plus the top end kit. I was thinking if it would be just as effective to bore it out to the next size up and get a piston kit to fit. 38A86458-27B5-4D9D-8DDF-B9FD86EB3187.thumb.png.615435955b0facd3cd7ee7c3c188895f.pngACA4CC24-577F-48CE-B339-529C62E6390C.thumb.png.2bc71b99e3185e7ac85bf351ddd52b4f.pngD0BE0F51-9F91-45A5-B9A1-C4E0D0E236D5.thumb.jpeg.57fe3d12f79a2ddbd8a2ca644de9c31e.jpeg
Thanks, Kodie 
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Not sure what Yamaha wants for the fuel ratio in that bike. I run 40:1 in my Kx 250 and it runs good. Two strokes love to go when you hold them wide. I’ve done it my self. Blew a yz 490 up in the desert at about 100mph , held it wide open for about 5 miles and that’s all it took. Get a new piston and have the cylinder honed. Should be good to go.

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The engine didn’t seize because it was run full throttle.  When properly set up it will run WFO with no problem.  Get yourself a shop manual.   Looks like you ran straight gas in it. 
 

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If it was me I'd go with a Eric Gorr YZ144 kit. I think it cost me ~$600ish and it includes boring, porting(I run mid-top) and head work with a forged Weisco single ring piston for a major performance increase. If jetted and oiled properly it will hold up well. I run "plenty" 26/28-1 of yamalube 2R and jet the main richer for higher speed desert riding. Plus with a 144 you can run higher gearing. Also remember that the "proper" warming up of forged pistons is "very" important.

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40 minutes ago, Tweider51 said:


Not sure what Yamaha wants for the fuel ratio in that bike. I run 40:1 in my Kx 250 and it runs good. Two strokes love to go when you hold them wide. I’ve done it my self. Blew a yz 490 up in the desert at about 100mph , held it wide open for about 5 miles and that’s all it took. Get a new piston and have the cylinder honed. Should be good to go.

And that’s the thing, the dude had the jets changed in his carb so I don’t know how I was supposed to run the bike. I’m so so new at this so I was trying to get everything back to stock so I could understand the changes in jets or fuel mixture and what gears to go through and when not to go into gears. I just started riding not even a year ago. I’m almost 19 and have no one to go to for questions other than the internet but I can’t even trust that half the time. I’m so infatuated with dirt bikes and just want to learn everything and fix everything the right way myself. I just want my bike running again and I don’t know how to care for it properly so everything is a trial and error to say the least. Thank y’all so much for the help. Means a lot 

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Looks like an owner issue. New -ish piston. Run WOT for too long, dry seize. You have a MX bike, designed for track riding, not flat out on long straights. You must change oil ratio, jetting and other things if you are just going to rip down the road. If you do not, this will happen again.

Reading through this post because his piston looked similar to mine. I have a 97 exc 300 which I’ve had for years and had rebuilt several times. My jetting has always seemed perfect to me but I have had several lean seizures. I suspect from air leaks. Where I ride and the people I ride with I can not avoid running near WOT. what can I do to make sure it’s not lean wide open (assuming I have no air leaks?). Motor is out being plated and rebuilt top and bottom and I want to make sure I get full life out of it. I run 42/1 castor 927 maxum pre mix. 42 pilot, 172 main jet. Should I increase main jet and run 32/1 to protect WOT? Sorry to hijack op thread but answers may be relevant to him as well.

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1 minute ago, Husqydude said:


Reading through this post because his piston looked similar to mine. I have a 97 exc 300 which I’ve had for years and had rebuilt several times. My jetting has always seemed perfect to me but I have had several lean seizures. I suspect from air leaks. Where I ride and the people I ride with I can not avoid running near WOT. what can I do to make sure it’s not lean wide open (assuming I have no air leaks?). Motor is out being plated and rebuilt top and bottom and I want to make sure I get full life out of it. I run 42/1 castor 927 maxum pre mix. 42 pilot, 172 main jet. Should I increase main jet and run 32/1 to protect WOT? Sorry to hijack op thread but answers may be relevant to him as well.

I’m in the same boat you are dude

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