new bike, yz125, blew it up

Joined
May 6, 2013
Location
Portland
Picked up my first dirt bike, rode it a handful of times and blew my top end. Can anyone tell what happened based on these photos? No idea on the prior owners maintenance or when the last time it was rebuilt.

IMG_0046.jpeg
piston.jpg
cylinder.jpg
 
Joined
Oct 18, 2016
Location
redding
looks like the ring cought the exhaust port, bore could have been out of spec, or maybe wasn't warmed up properly.
 
Joined
May 6, 2013
Location
Portland
Thanks for the replies, the crank seems fine but i am not an expert. it spins freely, no noises or grinding at all. looks like a chunk was taken out of the cylinder wall
 
Joined
Jul 12, 2017
Location
New Zealand
You're that far into it now, get the crank checked if you arent sure. Bottom ends dont always make noise but if its worn enough to give even a fraction of a mm movement it will end up being a bad, and very quick, first ride after the top end rebuild.
Was anything missing or broken when you pulled the piston out? Ring, clip? Something rattling around and could easly now be in the bottom end.
Nice bike, good luck sorting and getting back out soon.
 
Joined
May 6, 2013
Location
Portland
the piston ring looks like it caught on something and bent, wrist pin bearing looked fine, both clips in place and looked fine
 
Joined
Jun 9, 2015
Location
Australia
I have had a few YZ125s. Favourite bike would be the last of the steel frames for sure. Piston rings smashed up I would say.

My advice would be to have someone who is in the know look at the crank. If it checks out run it.

Have the cylinder sleeved also. Safer than running into more trouble with a plated cylinder.

I had a sleeved to 133 (?)cc 2003, mild porting, knife edging and clean up, with a pipe and honestly ran so good for so long. The plating is technically better, but if you are a casual rider or local racer it's not worth the extra 5% IMO.......
 
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