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I just recently purchased a 1996 cr 250 and the top end was replaced before I bought the bike so I put around 15 hours on the bike and one day I warm up the bike and 30 seconds into the ride my bike seizes solid and once I took the bike apart the crank was melted and I was wondering what could have caused this because I don't think it was over revving seeing I was only on the bike for 30 seconds.

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So the cylinder came readily off of the piston when you dis-assembled the motor?

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What do you mean the crank was melted?  The crankshaft is the single most robust part of your engine.  The cylinder and crankcases  would melt before the crankshaft!

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Some pictures here would be welcome.  I am going to guess that when the term "melted" is used,  it means that the lower rod bearing became so hot that it seized and deformed.   Terms like you find in  manual are much better when you describe what has happened.   (That is my round about way of saying that the OP needs a manual,  and needs to read it.)

As far as why it melted it can be as simple as it just got so worn that it finally generated enough heat to complete the job it had been working on for several rides,  and was likely about to do it at the end of it's last ride if not shut down when it did.  Lots of oil theories are likely to be repeated here too. 

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7 hours ago, ossagp said:

Some pictures here would be welcome.  I am going to guess that when the term "melted" is used,  it means that the lower rod bearing became so hot that it seized and deformed.   Terms like you find in  manual are much better when you describe what has happened.   (That is my round about way of saying that the OP needs a manual,  and needs to read it.)

As far as why it melted it can be as simple as it just got so worn that it finally generated enough heat to complete the job it had been working on for several rides,  and was likely about to do it at the end of it's last ride if not shut down when it did.  Lots of oil theories are likely to be repeated here too. 

Improper air/fuel mixture usually results in a stuck piston.  I can't think of anything, other than old age as you suggest, that would result in a stuck big end bearing.  Nice moniker by the way Ossa!

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Okay guy's, since nobody else has jumped in on this yet.  A seized two stroke engine is most often the result of a piston welding itself to the cylinder due to improper air/fuel/oil mixture.  This failure will reveal itself when you try to dis-assemble the engine and you can't get the cylinder off after removing all of the head and base bolts.  In fact the cylinder will move up and down if you try to rotate the engine via the kick starter.  If the cylinder comes off with minimal effort then the problem lies with in the bottom end of the engine.  This can range from a stuck big end rod bearing to stuck crank main bearings.  In order of likely hood the rod bearing is more likely than the crank main bearings.  There is one more thing that should be investigated before splitting the cases if you believe the bottom end has seized up.

 

In 1975 I had an RM-125 Suzuki that I had bought brand new from the dealer.  One day my brother and I were out riding and I let him ride the RM.  While he was riding it the engine locked up.  We loaded it up and took it home where I proceeded to take it apart.  Off came the cylinder head, exhaust and carb.  Then I unbolted the base nuts securing the cylinder to the crankcase.  To my great surprise the cylinder lifted right off with only the drag you would expect from the rings.  With the cylinder off the crankshaft still would not turn, so I started pulling the side covers off.  I pulled the cover of the stator off first and found that the nut securing the rotor of the generating system had backed off from the rotor and ground itself into the cover.  There wasn't enough clearance for the nut to back all the way off so it locked up the crankshaft.  When I went to the Suzuki shop they told me that there had been reports of the tang washer that was supposed to secure that nut failing on the early production bikes.  They gave me a new, improved washer along with the gaskets I needed to put the engine back together for free.  The bike ran fine for as long as I owned it after that.  The moral of that story is that mechanical failures other than bearings can result in a stuck crankshaft.

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I see seized rod bearing and crank bearings sometimes.  But usually seized is from piece of piston or something else.  Did a ktm 65 the other night had broken v force reed in bottom end.  Bottom rod bearing and crank bearings need oil and do not last with dirt being sucked in goes there first.  Usually the main cause I see with seized bearings. 

Edited by Motox367

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11 minutes ago, YHGEORGE said:

I want to see pics of a "melted" crank.

Seen alot of the plastic counter weights melted couple even broke.  But that engine doesn't have them lol that happens to every crank when goes bad . Hit from no oil, bad crank bearings causing lean, usually takes top first tho, dirt,sand don't take long they get hot.oh wide open road riders to. You know this ik just saying.  They appear melted to some people lol

Edited by Motox367
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15 hours ago, GlennRay said:

Improper air/fuel mixture usually results in a stuck piston.  I can't think of anything, other than old age as you suggest, that would result in a stuck big end bearing.  Nice moniker by the way Ossa!

Well i already knew a "Monte".  (FYI I still have a vr414 that is aching to have the tank put back on it.)  

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