YZ 125 (something went through motor)?

23champ
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23champ
Edited Date/Time 7/3/2022 2:23pm


This was in the exhaust pipe. I've checked the wrist pin bearing and all is good, I've checked the lower rod bearing and all is good, there are NO needles missing. Rod is tight up and down, and does not have excessive side play either . This entire bike only has about 5hrs on it since it was rolled off the dealer floor. Luckily the cylinder walls were not damaged. The head on the other hand is a different story. It's beat up pretty bad, as well as the top of the piston. The top of the piston smashed down and pinched the ring in the ring land reducing compression completely.

It's hard to tell in the photo but the piece that went through the motor is square in shape. It was not beat into a square either because it's a perfect square with perfect 90* angles on all sides.
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ianhendry46
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ianhendry46 5/7/2017 6:06pm
5/7/2017 6:06pm
Well I'm guessing since the spark plug is missing the electrode ground tang, it's related to that. If nothing else in the motor is missing and your only damage is to the head, that's probably it. I've seen it on a couple of new YZ250f's. It looks to me like the tang broke off, and was beaten strait. Odd, though. The tang breaking off, in my experience, is caused by a lean running condition
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23champ
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23champ 5/7/2017 7:19pm
5/7/2017 7:19pm
I'm guessing the ground strap on the spark plug got broke off by the piece beating around in the combustion chamber. If you look and the small silver specs on the head it's evidence that the piece was beating into the dome on end as well (basically stabbing). The ground strap evidently went on out the exhaust port. All this happened at or around max rpm. There was a 450 in front of me that I was trying to pass, so I couldn't hear what was going on inside the motor.

I'm quite certain the ground strap was not burnt off by a lean condition judging by the base ring of the spark plug. If anything it needs to be leaned up.

Paw Paw, I agree, it DOES look like a needle from the lower rod bearing. I checked the bearing well. I spun the bearing with a awl and look through the oiling slot in the bottom of the rod. No missing needles for one, and the piece is square instead of round as well. This has certinaly got me scratching my head.

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23champ
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23champ 5/7/2017 8:00pm
5/7/2017 8:00pm


This is how I checked the bearing. Scratch awl with a hook at the end. I was able to spin the bearing and inspect all the needles in the bearing. I must have rolled that bearing around 50x thinking I missed something.
TheLostOtter 7/3/2022 2:23pm
7/3/2022 2:23pm
Did you ever figure out the culprit? I just had this exact thing happen to my 2017 TC250 with 36 hours on it (5 hours since I did the topend). I found two pieces that look like the debris you found; one in the pipe and the other down in the crank case. Crank looks and feels perfect. Frustrated.

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