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2017 yz250 cylinder head failure destroys engine

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Head is cracked on combustion chamber and out side on top coolant went into the motor hydralic locked ruined, cylinder, piston, broke rod not sure about cases yet. 15 hrs of hard MX use since last top end was going to replace piston again this week. Cylinder head installed and torqued properly, head was true not warped. Is this common this is the 3rd one in the last 2 years at our local track. Kid rode it out as it locked up solid over a big down hill double. Head bolts seemed tight when i took it apart. So did the head oring fail causing coolant to go into the engine or did the head crack and cause this?

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Yzs have a history of being bomb proof... something definetly wasnt right and or something maintenance wise was neglected.

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Pictures?  If it all kind of happened at once it might be hard to say what the starting point was and the chain of events but clearly something wasn't right and needed attention, it didn't just crap out.  Did you notice anything at any point before and just said "ehhh screw it, ill just do this last moto and fix it later this week"  <--- this IMO is more often than not the starting point.  I may have been guilty of this once or twice during my riding career.  

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Bet it got hot and melted inner oring. Pic below was a 85 that happened to

20170504_095331.jpg

Edited by Motox367

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YZs are tough engines, known for their reliability. If this is the third time and the common factor is you, possibly something you are doing is causing this. Maybe not, but that's the first place I would look, at my own rebuild and maintenance procedures. Don't take this wrong. You sound like you absolutely know what you are doing. But maybe you are missing something simple or torquing something critical to the wrong value. Maybe the bike is running really lean somewhere that is not obvious.

Hopefully cases are good and you can rebuild. Entire top end, including cylinder, plus crank and all bearings is about $700, which you probably already know. If cases are good you have basically a new motor for $700.

Could just be bad luck. So I wish good luck!

Edited by LSHD
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33 minutes ago, LSHD said:

YZs are tough engines, known for their reliability. If this is the third time and the common factor is you, possibly something you are doing is causing this. Maybe not, but that's the first place I would look, at my own rebuild and maintenance procedures. Don't take this wrong. You sound like you absolutely know what you are doing. But maybe you are missing something simple or torquing something critical to the wrong value. Maybe the bike is running really lean somewhere that is not obvious.

Hopefully cases are good and you can rebuild. Entire top end, including cylinder, plus crank and all bearings is about $700, which you probably already know. If cases are good you have basically a new motor for $700.

Could just be bad luck. So I wish good luck!

read the post happened to 3 different people at our track first time to our bike. maintenance it top notch bottom end rebuilds at 40 hrs top ends every 15 hrs.

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2 hours ago, gedakbx said:

read the post happened to 3 different people at our track first time to our bike. maintenance it top notch bottom end rebuilds at 40 hrs top ends every 15 hrs.

Maybe you guys in Alberta are hard on the bikes, and track conditions require a differnt way to ride the bike. Sounds like a cold seize on the backside of a downhill. 

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read the post happened to 3 different people at our track first time to our bike. maintenance it top notch bottom end rebuilds at 40 hrs top ends every 15 hrs.

You must be a pro! Bottom ends every 40 hours? I know some really fast dudes and a couple pro riders who go hundreds of hours on a bottom end.

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4 hours ago, hondaman331 said:


You must be a pro! Bottom ends every 40 hours? I know some really fast dudes and a couple pro riders who go hundreds of hours on a bottom end.

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Pro mx going hundreds of hrs on a crank I doubt it. 

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Was no pro but was pretty good local A racer before I took break few years ago.  Now I'm just a good vet B racer with spurts a A speed I feel lol anyway after 125s i raced kx250s then yz250's only had one crank failure but 4 tranny failures all yz's :thumbsdn: I did top and bottom once a year that's it. Never heard of this problem before tho something is fishy here:ride:

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Pro mx going hundreds of hrs on a crank I doubt it. 

Believe it. A 2001 cr125 and a 2001 cr250. It's a 2 stroke, cranks go way longer then on a thumper and he beats those bikes. He races a 450 Honda, new every year and they are junk at the end of each season.....but rides the piss out of those smokers bone stock.

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4 hours ago, P84ONE said:

Maybe you guys in Alberta are hard on the bikes, and track conditions require a differnt way to ride the bike. Sounds like a cold seize on the backside of a downhill. 

Maybe 25 mins into a 30 min moto cold seize is unlikely especially since the piston wasn't stuck at and the rod is bent and broke into 4 pieces

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5 minutes ago, hondaman331 said:


Believe it. A 2001 cr125 and a 2001 cr250. It's a 2 stroke, cranks go way longer then on a thumper and he beats those bikes. He races a 450 Honda, new every year and they are junk at the end of each season.....but rides the piss out of those smokers bone stock.

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Maybe a Honda not a yz 250. 

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Maybe a Honda not a yz 250. 

Funny you say that, I also know 2 isde junior trophy team racers, (a brother and sister so you can figure that out), and the torture they do to bikes is just disgusting. Of all the bikes they've raced over the last 20 or so years the yz has proven the most durable. 2 stroke cranks last a long long time, it isn't a 4 stroke. I have never replaced any of the cranks in my 2 strokes over the last 20+ years, have done plenty on 125s after massive blow ups or melt downs, but a 250.....very rare. 4 strokes are a different story, my 08 450 Honda couldn't get past 125 hours and it would need one. My current yz450 is at 86 hours since coming off the showroom with nothing but typical maintenance. If a person doesn't clean their filter or does so and drops dirt nuggets in the airboot, yeah, a smoker will go through main bearings, but what you are saying is foreign.

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2 minutes ago, hondaman331 said:


Funny you say that, I also know 2 isde junior trophy team racers, (a brother and sister so you can figure that out), and the torture they do to bikes is just disgusting. Of all the bikes they've raced over the last 20 or so years the yz has proven the most durable. 2 stroke cranks last a long long time, it isn't a 4 stroke. I have never replaced any of the cranks in my 2 strokes over the last 20+ years, have done plenty on 125s after massive blow ups or melt downs, but a 250.....very rare. 4 strokes are a different story, my 08 450 Honda couldn't get past 125 hours and it would need one. My current yz450 is at 86 hours since coming off the showroom with nothing but typical maintenance. If a person doesn't clean their filter or does so and drops dirt nuggets in the airboot, yeah, a smoker will go through main bearings, but what you are saying is foreign.

Shit, all I've ever done with 2t cranks is have them rebalanced, inspected, and rebuilt if necessary.

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Shit, all I've ever done with 2t cranks is have them rebalanced, inspected, and rebuilt if necessary.

I check for play and then put it back together. Wrist pin bearings, less engine decompression and less rpms make 2 stroke cranks last much much much longer then 4 strokes. Some people like wasting money though I guess.

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8 hours ago, gedakbx said:

read the post happened to 3 different people at our track first time to our bike. maintenance it top notch bottom end rebuilds at 40 hrs top ends every 15 hrs.

Do the 3 different people at your track all use the same engine builder?

 

15 hrs on a top end when Yamaha says 50, and 40 hrs on a bottom, what the hell?

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The original post mentions 'downhill double'.

Would this particular track configuration make for a lot of compression braking on a long downhill ?

(forced high rpms with the throttle closed, lean surging the engine)

Edited by mlatour
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2 hours ago, mlatour said:

 

The original post mentions 'downhill double'.

Would this particular track configuration make for a lot of compression braking on a long downhill ?

(forced high rpms with the throttle closed, lean surging the engine)

To clarify the engine locked up due to the debri from the broken rod (in at least 4 pieces and bent also) piston skirt, and lower rod needle brg and cage being jammed in between the crank journals and the case. No evidence of the pistion sticking to the cylinder there was coolant in the pipe and bottom end when i took it apart and the head is cracked (chicken or egg that is what i am trying to figure out haha) 

track is the calgary pro national track track is fast and flows well my son has road it for 8 years on yamha yz 85/105/112/125/134/and 250 2 strokes he rides 150 hrs a year plus

rider is a 15 year old intermediate who qualified for all 4 western rounds of the pro nationals and did well enough at Calgary and Regina to earn a top 100 pro number for next year number 84 to be exact. He also won the 2018 yamaha factory ride at the Grand National amatuer championship a month ago In Kamloops. He wears out bikes like crazy.

we have a 2016 that has been bullet proof rebuilt crank at 100 hrs and change pistons at 30 hrs this is the practice bike.

the 2017 is his race bike crank went at 25 hrs lower rod brg air filters are changed after every moto or 2 and the intake boot is spotless. Found metal on the drain plug at 45 hrs pulled apart and found a couple bad gears. Now this at 59 hrs i think this bike is just a lemon. He is loosing faith in this bike and will finish the last 4 race days on his old bike. Both bikes are set up the same except the 2017 has the STIC in the carb and is jetted accordingly

 

thanks

 

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