Help - CR125 Keeps Eating Pistons - Why?

Help.  My CR125 was running ok all summer until I the bike and I went down going around a turn.  the bike revved for 5-7 seconds before I could shut it off.  The bike was laying on it's side.  When I stood the bike back up and tried to start it, it had no compression.  I got it home and took the head off.  The piston had a hole in it and the Cylinder wall was trashed and the cylinder head was trashed.  I also noticed the power exhaust valves (RC valves) were "stuck" in the full open position, from carbon buildup.  So the bike was running this way for likely many months or longer.   I bought a new cylinder jug, head, piston kit (standard 54mm), power exhaust (RC Valves), and gasket kit.  Put it all back together with the piston installed per instructions and so on.  Started it up.  Did a break in procedure (ran for 10 minutes at idle, then shut it down to cool off).  did this 2 or 3 times.  Took the bike out for a ride up and down my neighborhood streets.  It ran ok, but I could tell something was wrong.  It had low end power but mid and high rpm range it ran poorly.  Then it happended.  The bike died out at mid to high rpm in 3rd gear.  Had to walk it home about 1/2 mile.  Took it apart.  The top of the piston was damaged and the head was damaged.  Cylinder walls were ok, power exhaust (RC valves) were ok.




A couple of things you need to know:


  1. when I put the motor back together and went to start it up (with all new parts) I noticed the compression was low.  I did not take a reading, but by kickstarting it, I noticed it was much lower than it was since I owned it.  This concerned me very much, but this was my first 2-stroke rebuild and I thought it might be normal for a new piston/cylinder install prior to break in. 
  2. blows lots of smoke - but it always did and ran fine before
  3. i did not check, or replace the Reed Valve - ever!
  4. the piston (Wiseco standard) was indeed installed correctly (had an arrow to indicate which way to install)
  5. i used a torque wrench for all cylinder and head bolts/nuts
  6. the radiator was full and the water pump appeared to be working because the hoses were getting warm when I was running it during the break in (sitting on the stand idling)
  7. I did not check or even think about the "bottom end"  Never looked at or touched or inspected any of that.
  8. running 32:1 mixture - standard 87 octane gas (normally works fine)
  9. carb has been rebuilt by me recently and is good, has standard jetting and settings.  Float level is good.
  10. running standard spark plug
  11. the power exhaust valves (new) appear to be installed and working properly but I don't know for sure.  I can move them by hand with the bike off by pushing/pulling the control rod from the governor assembly.  There is also a possibility that I did not install the power valve assembly correctly.  There are c-clips, washers, a couple of range limiters and a spring that all go together on the end of the rod.  I'm 75% sure I did it right, but not 100%.  I compared it to photos I took and assembly diagrams and it seems to be right, but I'm not sure.



thanks for reading this and thanks for your help.  Lee



Edited by chevywaldo

I would start by doing all of the things you didn't do.

Why is it eating pistons? Because it's hungry! 


Now, in light of a crappy situation...
It looks like something bounced around in the combustion chamber. Find out what it was. Did you check the ring end gap? Im more then willing to bet this is the issue since you didn't have proper compression when you kicked it over for the first time. Maybe you broke a ring when installing it the piston? 
Sounds like your power valve assembly isn't might not be working correctly. Check the reeds, as they could influence a lot of what you discussed. 
How long before you burned a hole in your first piston did you rejet?

Looks like rod bearings to me either big or small end

Looks like rod bearings to me either big or small end


Thats what I was thinking based on the damage to the head... But Im no mechanic. 

Words of advice given to me from my father long ago:


Anytime you have a top end failure you need to split the case to clean out the big end bearings.  Small debris will get in the bearings and will work their way out after you start running the motor again.



Whether or not the bearings need replaced I couldn't tell you, but definitely split the case and clean everything when you rebuild it next time.

I am no mechanic so I may be completely wrong here but first impression is the damage on the top of that piston must have come from either

A) something major that came in with the fuel (Unlikely) or

B) something that was already in the bottom end.


The dents in the piston and head are inconsistant with the minor scratches on the side of the barrel.


I had a mate who had a very similar thing but he had installed his piston backwards so it snapped the rings as they caught on the ports. I know you said you checked that already.

I hope you get it sorted, I know how frustrating and expensive it can be. Good luck.



thanks for your help.  I did install the correct piston (with new pin and bearings) and I did install it correctly and I did check and correct ring end gap.  That is all good.  I'm thinking there was metal pieces still in the crankcase from the prior blown piston, and that's what got sucked into the cylinder and did the damage.  I agree that the case has to come apart and be rebuilt, cleaned, bearings checked and replaced if needed, etc.  Having said that, I do  have a service manual and it appears that splitting that case apart is a big and very detailed technical job requiring specialty tools, lots of time, and perhaps some experience.  I'm a little concerned I'm getting in over my head here.  Doing a cylinder jug and piston was easy, but pulling the case apart seems like climbing a mountain.


Is there a way I can open up the case and check what I need to check, and clean it all out without doing an entire case rebuild?  I may screw it up, waste 20 hours doing it, make 1 tiny mistake, and blow the engine again, costing me hundreds of more dollars.  I've already spent $500.  The entire bike is only worth about $900.  I can buy another used bike in working condition for $900.  I'm wondering if I even want to tackle this job, or just sell the bike as is for a few hundred bucks and buy another bike.


Can one of you guys tell me if this is a job that I can do successfully or will it require trained 2-stroke bike mechanic with specialty tools to do the job?  I'm pretty handy and have rebuilt V8 car engines, but this bike crankcase seems like it's more complex than a car engine.


Should I attempt to rebuild it and pour time and money into it, or just get another bike?


thanks.  Lee

If you can build a v8, you can surely build this. Before tearing it down, I would pull the stator cover and grab the flywheel, and see if it has any up and down play. If not, the crank bearings should be good. I would also check the big end ( lower rod bearings ). If all checks out ok, simply tear it down, and clean it out. Be out a center gasket, but you can replace anything else you may want to at that time. Me, If I tore it down that far, I'd do main bearings for piece of mind regardless. I say pull it down, and fix it. 900 for another just might need 500 more soon.

If you have a number of other problems with the bike then I would sell it but if all else is ok then fix it and ride it.

I just finished my first bottom end and like you have done a few engines for cars. It seemed intimidating but really it isn't that bad. One tip, keep the tranny gears and washers together the best you can. I got 2 washers mixed on the counter and main shaft for the trans and split the case 6 times trying to get it right. But I got it. 


With the cost of repairing it, and just buying another bike it might be better to just get another bike. I was kind of at the point you were at and thinking the same thing but I would rather have a bike with new parts and know I don't have to replace them instead of buying one and not knowing when or who did the work on it last. 

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