Kx125 Motor Rebuild (exploded piston)

devenchilton
Edited Date/Time 4/3/2020 9:57pm
Hey everyone I have a 2001 KX125 that seized up a couple years ago and has just been sitting since. I finally got around to pulling it apart and once got the top end off found the piston was broken, and the crank was still seized. Looking down Into the bottom end I could see chunks of metal that fell down and all sorts of gunk and grime. Also I noticed that there is a decent amount of rust on the crank. Since I don’t have a splitter and haven’t ever done any work on the bottom end before I decided to try to clean out all the metal chunks and shavings by flushing out the bottom end with gas and oil. Did that about 15 times and managed to get the crank turning over freely and smoothly.

So my first question is, do you guys think I’m good to get a new piston and go ahead and rebuild the top end? Or is it stupid to do that without actually splitting the case and making 100% sure there aren’t any metal chunks left over?

My second question is will the rust effect anything? Obviously replacing the crank would be ideal but I don’t want to sink a whole lot of time and money into such an old bike.

Thanks so much, I will try to get some better pictures tomorrow.
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Paul_Pitzonka
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Paul_Pitzonka 3/28/2020 8:42pm
3/28/2020 8:42pm
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end rod bearing looks like) Also I doubt with that much damage to your piston your bore is in good shape...
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devenchilton 3/28/2020 11:17pm
3/28/2020 11:17pm
Paul_Pitzonka wrote:
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end...
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end rod bearing looks like) Also I doubt with that much damage to your piston your bore is in good shape...
Thank you for your help Paul. You think the whole crank would need to be replaced? Do you have any idea how much a shop would charge to fix it up?
navyslideby
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navyslideby 3/29/2020 9:31am
3/29/2020 9:31am Edited Date/Time 3/29/2020 9:35am
Ya, that whole motor needs to come apart for sure man..... I looked and I cant find a wrench rabbit kit or a hot rods kit for that bike... I guess they are not made.... So you would be piecing things together. At a shop, with shop labor rates, you are probably looking at a couple thousand bucks with parts and labor mate.... doing it yourself would be ideal, but you obviously need the tools and the knowledge.

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CarlinoJoeVideo
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CarlinoJoeVideo 3/29/2020 11:51am
3/29/2020 11:51am
Paul_Pitzonka wrote:
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end...
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end rod bearing looks like) Also I doubt with that much damage to your piston your bore is in good shape...
devenchilton wrote:
Thank you for your help Paul. You think the whole crank would need to be replaced? Do you have any idea how much a shop would...
Thank you for your help Paul. You think the whole crank would need to be replaced? Do you have any idea how much a shop would charge to fix it up?
I just looked on eBay and see brand new cranks for $250 and top end kits for $150. If you get all new deals and gaskets. The parts will be about $500-$750 total and if you need extra tools. This also depending on how OCD you are.
devenchilton 3/29/2020 1:18pm
3/29/2020 1:18pm
Paul_Pitzonka wrote:
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end...
That needs to come completely apart there is way too much rust in that engine (I can’t imagine what the main bearings or the big end rod bearing looks like) Also I doubt with that much damage to your piston your bore is in good shape...
devenchilton wrote:
Thank you for your help Paul. You think the whole crank would need to be replaced? Do you have any idea how much a shop would...
Thank you for your help Paul. You think the whole crank would need to be replaced? Do you have any idea how much a shop would charge to fix it up?
CarlinoJoeVideo wrote:
I just looked on eBay and see brand new cranks for $250 and top end kits for $150. If you get all new deals and gaskets...
I just looked on eBay and see brand new cranks for $250 and top end kits for $150. If you get all new deals and gaskets. The parts will be about $500-$750 total and if you need extra tools. This also depending on how OCD you are.
Sweet thanks man! Honestly I just wanna get this thing running for some rides this summer, not super OCD about the engine being perfect. What specialty tools will I need? Just a case splitter?
navyslideby
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navyslideby 3/29/2020 5:33pm
3/29/2020 5:33pm Edited Date/Time 3/29/2020 5:34pm
Case splitter, crank puller, flywheel puller. Could potentially need some blind bearing pullers. And aside from that basic hand tools, socket set and Torque Wrench.... if you are gonna pull all of that out, I really recommend you replace all the bearings and seals while you are in there. Go to rocky mountain, OEM parts, and type in your bike info. Then find the breakdown for the crankcase and get all the bearings and seals you can see... not too much expense added but a lot of peace of mind.

You also REALLY need to inspect that cylinder very closely or better yet have a local machine shop check it out and make sure there's no damage to it....
Brett Campbell 3/30/2020 5:21am
3/30/2020 5:21am
Youtube videos are a great learning tool, since you have never rebuilt a bottom end.
2F4A
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2F4A 4/2/2020 4:08pm
4/2/2020 4:08pm
Cylinder? If not repairable look for one before you order anything else. I have been looking for ages for new and used cilinders and it took a long time before I finally got them.
project racer 4/2/2020 5:16pm
4/2/2020 5:16pm
2F4A wrote:
Cylinder? If not repairable look for one before you order anything else. I have been looking for ages for new and used cilinders and it took...
Cylinder? If not repairable look for one before you order anything else. I have been looking for ages for new and used cilinders and it took a long time before I finally got them.
Very good suggestion. That would definitely hold back progress.
devenchilton 4/2/2020 8:15pm
4/2/2020 8:15pm
2F4A wrote:
Cylinder? If not repairable look for one before you order anything else. I have been looking for ages for new and used cilinders and it took...
Cylinder? If not repairable look for one before you order anything else. I have been looking for ages for new and used cilinders and it took a long time before I finally got them.
As far as I can see there isn’t any damage in the cylinder, just some corrosion. Do you think I’d be able to clean it up with a flap wheel or something like that on a dremel? On the bottom the edge is a little bit rough, not sure if that is damage or not.O



timobijnen84 4/2/2020 11:28pm
4/2/2020 11:28pm
Dont use a flap wheel youre cilinder will be ruined. And youre cilinder walls dont look that great either. Find a used one or get this one sent out for nicasil coating but that is also expensive. It will be around 225 dollars but you will not regret it.
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2F4A
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2F4A 4/3/2020 1:29am
4/3/2020 1:29am
That cilinder needs to be replated.
project racer 4/3/2020 9:21am
4/3/2020 9:21am
If the rest of the bike is not in good condition, you will have more in it than it is worth by the time it is all straightened up. No sense in throwing good $$ after bad. Pull that thing down for the full rebuild, or part it on ebay.
devenchilton 4/3/2020 3:38pm
4/3/2020 3:38pm
timobijnen84 wrote:
Dont use a flap wheel youre cilinder will be ruined. And youre cilinder walls dont look that great either. Find a used one or get this...
Dont use a flap wheel youre cilinder will be ruined. And youre cilinder walls dont look that great either. Find a used one or get this one sent out for nicasil coating but that is also expensive. It will be around 225 dollars but you will not regret it.
What about scotch brite pad? Could those microscopic scratches really effect it that much? The pictures look worse than it is, when you run your fingers over you can barely feel it. What would happen if I didn’t get it re plated? Cause over heating? Thanks everyone for takin the time to help me
Falcon
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Falcon 4/3/2020 4:21pm
4/3/2020 4:21pm
Brett Campbell wrote:
Youtube videos are a great learning tool, since you have never rebuilt a bottom end.
Youtube videos are a great learning tool, since you have never rebuilt a bottom end.
This. I just recently learned how to disassemble forks thanks to Youtube. Now I can rebuild the Space Shuttle.
project racer 4/3/2020 9:54pm
4/3/2020 9:54pm Edited Date/Time 4/3/2020 9:57pm
devenchilton wrote:
What about scotch brite pad? Could those microscopic scratches really effect it that much? The pictures look worse than it is, when you run your fingers...
What about scotch brite pad? Could those microscopic scratches really effect it that much? The pictures look worse than it is, when you run your fingers over you can barely feel it. What would happen if I didn’t get it re plated? Cause over heating? Thanks everyone for takin the time to help me
If it broke the skirt on the piston ,that is a good sign of loose clearances. When they get loose, piston rocking sets in, and skirt cracks.

Rinse it out, throw a scuff and a piston at it, and see what happens. Low cranking pressure is one concern on a loose bore. Rust on the rod / main bearings usually leads to short life.

Edit: The stray metal / rinse deal is iffy, at best.

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