CR125 Piston Scuffing?

I just bought a 2002 cr125 and I was planning on rebuilding it at the end of this season but I'm thinking that I should do it earlier.  It has 145 psi compression and it was last rebuilt in 2014 by the previous owner.  The bottom and top end was done along with a new cylinder.  Apparently it wasn't run much but I don't know.  I just took the pipe off today to look at the piston and it looked good but there was a weird scuff pattern on it.  It wasn't scratched or scored, just shiny.  The cylinder looked really good and I could clearly see the cross hatching.  Should I replace this piston now or does it still have life left?  What could have caused the scuffing?  Is it bad?

 

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That exhaust flange looks weird. Is that an aftermarket flange?

The best way to know if you need to replace the piston is to measure the piston to bore clearance.

That exhaust flange looks weird. Is that an aftermarket flange?

The best way to know if you need to replace the piston is to measure the piston to bore clearance.

It could be aftermarket but I don't know. Is there a reason it scuffed like that? Is this safe to run?

The piston "looks" ok to run. The proper way to check is to measure it.

There is a little "blow by" below the rings so a new ring would be nice but it's not the end of the world.

The exhaust flange looks like something boyesen offered no big deal...just an observation.

The piston "looks" ok to run. The proper way to check is to measure it.

There is a little "blow by" below the rings so a new ring would be nice but it's not the end of the world.

The exhaust flange looks like something boyesen offered no big deal...just an observation.

It probably is boyeseen because there's some other boyeseen parts on the bike too.

Thanks for the help

That exhaust flange looks weird. Is that an aftermarket flange?

The best way to know if you need to replace the piston is to measure the piston to bore clearance.

The exhaust flange looks weird cause its put on wrong. That cylinder has a flat section at the top of the port and the flange is the same. They have the flange installed with the flat section towards the right and not matched to the top of the cylinder.

Remove the 3 8mm bolts and rotate the flange to match the cylinder and i bet you the bike will run even better since the flange wont be blocking part of the exhaust port anymore.

It probably is boyeseen because there's some other boyeseen parts on the bike too.

Thanks for the help

Read what i posted above. Its the stock exhaust flange you just have it installed wrong. It needs lined up to match the port shape of the cylinder. The flat section on the right of the photo needs to be at the top of the cylinder.

Read what i posted above. Its the stock exhaust flange you just have it installed wrong. It needs lined up to match the port shape of the cylinder. The flat section on the right of the photo needs to be at the top of the cylinder.

I'll look at it tonight. Won't the springs for the pipe line up weird once I turn it? The springs line up perfectly right now.

I'll look at it tonight. Won't the springs for the pipe line up weird once I turn it? The springs line up perfectly right now.

The springs will still be fine. Worry more about getting the flange configured right with the exhaust port so it will breath better and the exhaust can flow out more freely.

The springs will still be fine. Worry more about getting the flange configured right with the exhaust port so it will breath better and the exhaust can flow out more freely.

It does run a little rich and it fouls plugs. Maybe it will stop if I fix it. Does anyone have a picture to make sure it's installed wrong?

I'm not familiar with that assembly but anything blocking the exhaust like that isn't normal,

as other have mentioned try to rotate the flange in another position.

 

The piston shows a lot of scuffing, perhaps rebuilt in 2014 but unknown hours of use since.

145psi is low, in the 160-170psi would be considered average for most 125's.

 

At the cost of a only few gaskets, I'd take it appart and have a look inside.

Finding a new piston installed in a worn out cylinder / plating wouldn't surprise me.

Edited by mlatour

It does run a little rich and it fouls plugs. Maybe it will stop if I fix it. Does anyone have a picture to make sure it's installed wrong?

I will take a picture when i get home in about 40 minutes to ease your mind but i can tell you its on wrong im 110% sure. I build alot of these motors and that flange is 90 degrees off from matching the cylinder port. Yes with the port blocked off like that it will run alittle richer since its not getting all the exhaust out.

I will take a picture when i get home in about 40 minutes to ease your mind but i can tell you its on wrong im 110% sure. I build alot of these motors and that flange is 90 degrees off from matching the cylinder port. Yes with the port blocked off like that it will run alittle richer since its not getting all the exhaust out.

Thanks! I know it's probably on wrong I'm just getting sick of taking this bike apart. I just put it all back together after having some problems with dust getting past into the air boot but this sounds like a simple fix. Thanks for getting the picture I'll change it tonight.

Thanks! I know it's probably on wrong I'm just getting sick of taking this bike apart. I just put it all back together after having some problems with dust getting past into the air boot but this sounds like a simple fix. Thanks for getting the picture I'll change it tonight.

Yes is a simple fix just remove the pipe and remove the three 8mm bolts and turn the flange 90 degrees. Use some honda bond or some high temp sealant on it. They are usually a common area for air leaks. Tighten the bolts back down and put the pipe back on. You might need to replace the rubber seal on the outside of the flange if its all chewed up that way the motor is good and sealed up. This picture is from a 2002 cr125 that i just completely rebuilt top and bottom for a customer.

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Yes is a simple fix just remove the pipe and remove the three 8mm bolts and turn the flange 90 degrees. Use some honda bond or some high temp sealant on it. They are usually a common area for air leaks. Tighten the bolts back down and put the pipe back on. You might need to replace the rubber seal on the outside of the flange if its all chewed up that way the motor is good and sealed up. This picture is from a 2002 cr125 that i just completely rebuilt top and bottom for a customer.

Thanks for getting the picture

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