Blown up cr125 bottom end question

jaaaaaaaaaaaaaaaaaaaaaaack
Edited Date/Time 10/28/2020 9:17am
I have a 2007 cr125 that I just blew the piston up on. I had been riding it pretty consistently for a couple months without any problems and it seemed to go out of nowhere. And when I took it apart i found that the piston was installed the wrong direction, so the ring caught on the exhaust port and ripped it up pretty good. What I'm wondering is if I need to do the bottom end on my bike. The piston is pretty thrashed, but only on the top 1/4. There was a good amount of piston bits in my expansion chamber. There are some small specks of aluminum in the crankcase, and a couple small pieces on my drain plug magnet, after running some more transmission oil through it no more metal shows up on the magnet. My crankarm doesn't have any up/down play and the side to side is in spec and everything moves smoothly. I am getting the cylinder bored out and a new head. Am I going to be alright throwing a new top end on, or do I need to split the cases and clean out and redo the bottom?



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Wolfey
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Wolfey 10/27/2020 5:25pm
10/27/2020 5:25pm
I know if it was mine,I would split it and go through it properly. It’s better to be safe and get it while it’s tore down.
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jaaaaaaaaaaaaaaaaaaaaaaack 10/27/2020 5:28pm
10/27/2020 5:28pm
Wolfey wrote:
I know if it was mine,I would split it and go through it properly. It’s better to be safe and get it while it’s tore down.
At that point would it make sence to do a whole bottom end rebuild, rather than open it up and clean it and put it back together?
Wolfey
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Wolfey 10/27/2020 5:34pm
10/27/2020 5:34pm
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a long time.
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jaaaaaaaaaaaaaaaaaaaaaaack 10/27/2020 5:41pm
10/27/2020 5:41pm
Wolfey wrote:
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a...
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a long time.
Yea that seems like the proper way to do it. Thanks for the advice
jaaaaaaaaaaaaaaaaaaaaaaack 10/27/2020 5:50pm
10/27/2020 5:50pm
Wolfey wrote:
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a...
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a long time.
Are there any special tools I need to do the bottom end? Theres some kinda flywheel puller/install tool correct?
JMCR250
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JMCR250 10/27/2020 6:10pm
10/27/2020 6:10pm
On a two stroke, the lower end is sealed off from your transmission, so if you're finding pieces of metal in your transmission drain plug they are coming from your transmission or clutch OR you blew a hole in the case between where the crankshaft rotates and your transmission. You have significant enough damage to the cylinder/piston/ring that you should split the cases and check the crank, crank bearings and the integrity of the cases. At the same time, maybe you'll be able to find out what is causing the metal bits on your transmission oil plug. You'll need a case splitter and a flywheel puller.

This is not a job for someone new to motorcycle engine work, as the transmission/shift/clutch assemblies are full of little parts and pieces that must go together in a very precise way. If you do try it, be sure to get a factory service manual and do a lot of research (e.g., You tube videos, etc.)

I'm just curious, how long did the bike run with the piston in backwards?

Good luck.
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Wolfey
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Wolfey 10/27/2020 6:11pm
10/27/2020 6:11pm
Yeah flywheel puller is one of the tools. You don’t have to have one but a crank splitter makes the job much easier. If it’s your first time doing a bottom end, you can watch some how to videos on YouTube. I know Rocky Mountain atv has some great videos on rebuilds. Good luck to ya.
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mxracer666
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mxracer666 10/27/2020 6:15pm
10/27/2020 6:15pm
Wolfey wrote:
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a...
Yep . It’s pretty simple and not that expensive on a 125. Then once it’s completed you will have a complete rebuild that should last a long time.
jaaaaaaaaaaaaaaaaaaaaaaack wrote:
Are there any special tools I need to do the bottom end? Theres some kinda flywheel puller/install tool correct?
Lots of special tools...and knowledge. Crankcase splitter, flywheel puller at minimum... Watch some youtube videos, to get an idea of what is required, and see if that matches your skillset.
jaaaaaaaaaaaaaaaaaaaaaaack 10/27/2020 6:31pm
10/27/2020 6:31pm
JMCR250 wrote:
On a two stroke, the lower end is sealed off from your transmission, so if you're finding pieces of metal in your transmission drain plug they...
On a two stroke, the lower end is sealed off from your transmission, so if you're finding pieces of metal in your transmission drain plug they are coming from your transmission or clutch OR you blew a hole in the case between where the crankshaft rotates and your transmission. You have significant enough damage to the cylinder/piston/ring that you should split the cases and check the crank, crank bearings and the integrity of the cases. At the same time, maybe you'll be able to find out what is causing the metal bits on your transmission oil plug. You'll need a case splitter and a flywheel puller.

This is not a job for someone new to motorcycle engine work, as the transmission/shift/clutch assemblies are full of little parts and pieces that must go together in a very precise way. If you do try it, be sure to get a factory service manual and do a lot of research (e.g., You tube videos, etc.)

I'm just curious, how long did the bike run with the piston in backwards?

Good luck.
The main crank bearing seems fine, just from pulling on the crank arm. And i had it for a couple months before it blew, allot longer than I would imagine it lasting backwards.
jaaaaaaaaaaaaaaaaaaaaaaack 10/27/2020 6:35pm
10/27/2020 6:35pm
The reason I am really asking is because I was planning to sell the bike and get a 250. And then the piston blew. So I'm trying to save money on the rebuild if possible. I dont want to put 800 bucks in for the new cylinder and top end and bottom end just to sell it.
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walent215
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walent215 10/27/2020 8:12pm
10/27/2020 8:12pm
Top end is all that is needed from what your describing.
Big end side play is within spec and there is no debris in the bottom end.
Run it.
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DynoDan22
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DynoDan22 10/27/2020 9:05pm
10/27/2020 9:05pm
I'm surprised it ran as long as it did. Wow.
mattyhamz2
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mattyhamz2 10/27/2020 9:06pm
10/27/2020 9:06pm
I’m with the guys saying split the cases. You never know how much metal went into the bottom end. Maybe none, but maybe some and that COULD cause you to have to go through it all over again.

I’m one of those that would rather be safe than sorry.
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jaaaaaaaaaaaaaaaaaaaaaaack 10/28/2020 5:48am
10/28/2020 5:48am
mattyhamz2 wrote:
I’m with the guys saying split the cases. You never know how much metal went into the bottom end. Maybe none, but maybe some and that...
I’m with the guys saying split the cases. You never know how much metal went into the bottom end. Maybe none, but maybe some and that COULD cause you to have to go through it all over again.

I’m one of those that would rather be safe than sorry.
When I look down into the crank I can see to what I would think is the bottom of the crank case and there doesn't seem to be any metal I can see, but there are other spots in the crank case that pieces could be that I cant see unless I split the cases correct?
jaaaaaaaaaaaaaaaaaaaaaaack 10/28/2020 5:51am
10/28/2020 5:51am
walent215 wrote:
Top end is all that is needed from what your describing. Big end side play is within spec and there is no debris in the bottom...
Top end is all that is needed from what your describing.
Big end side play is within spec and there is no debris in the bottom end.
Run it.
But dont I have to split the case to see if there is metal in the bottom end? If I rotate the crank I can see down the middle of the case and I dont see anything other than a couple little dust size specs, but from what everyone else is saying it sounds like I cant actually tell what's in there unless I slip the cases
timobijnen84 10/28/2020 6:32am
10/28/2020 6:32am
There is debris in the bottom end so split the cases and clean them very wel and check every bearing.
I have never used a case splitter just some flat bar and drill holes.
walent215
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walent215 10/28/2020 6:48am
10/28/2020 6:48am
walent215 wrote:
Top end is all that is needed from what your describing. Big end side play is within spec and there is no debris in the bottom...
Top end is all that is needed from what your describing.
Big end side play is within spec and there is no debris in the bottom end.
Run it.
jaaaaaaaaaaaaaaaaaaaaaaack wrote:
But dont I have to split the case to see if there is metal in the bottom end? If I rotate the crank I can see...
But dont I have to split the case to see if there is metal in the bottom end? If I rotate the crank I can see down the middle of the case and I dont see anything other than a couple little dust size specs, but from what everyone else is saying it sounds like I cant actually tell what's in there unless I slip the cases
If there was significant amount of debris in the bottom and yes you need to split the cases but if there was a couple minor little chips or flakes that you could see and remove them I would remove them and to be doubly sure I would turn the motor upside down rotate the crank and flush it out by spraying it with some WD 40 or solvent or something of that nature.
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FGR01
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FGR01 10/28/2020 8:02am
10/28/2020 8:02am
DynoDan22 wrote:
I'm surprised it ran as long as it did. Wow.
Yeah.. and I seem to recall the CR125 has a pretty sizeable window in the piston on the intake side. So I can only imagine how it was running with it backwards.

To the OP, you are correct that you cannot properly inspect the bottom end for debris without splitting the cases. The crank may be spinning fine now but there could very easily be an unseen debris that will move and lodge itself in a main bearing or migrate to the top end upon startup or running. If you're trying to be cheap, at the very least, remove the bottom end from the frame, fill the crankcase with kerosene or pre-mixed gas, shake and dump. Repeat this a few times. This might remove any small debris that's down in there. Also run a pen magnet around down in there in case there are any small bits of steel.
Paw Paw
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Paw Paw 10/28/2020 8:53am
10/28/2020 8:53am
Do it right! Don't shift some time bomb off on another rider as if it fails it can cause injuries.

Paw Paw
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jaaaaaaaaaaaaaaaaaaaaaaack 10/28/2020 9:17am
10/28/2020 9:17am
Thanks for all the advice, since I cant be sure that the bottom end is fine I'm gonna hold on to the bike for another year at least and just rebuild and clean up the whole bike. It's about winter time her in CO anyway so good time to tear it apart.
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