motor help~~~~

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#1
Well the nasty happened~~My 05 crf250r motor has decided to give it up.I did a top end about 3 months ago with new valves,new piston and rings,new clutch with basket and hub.Well according to a mechanic friend he said it looked like the piston broke where it connects with the rod,thus the piston hit the head.rod broke,and the part that was still connected to the crank decided to go through the front of the motor.Taking out the balancer shaft and both cases.My question is this: if this happened to you did you rebuild or replace the engine.I have worked up a parts list and have been on **** does anyone else know where I can find like used cases or any other parts,and also does anyone offer engines.I know this going to be expensive but I trying to lessen the pain~~~~Thanks for any advise
 

Rich Rohrich

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#2
Did the engine let go when the throttle was closing, or in a whoops section where you were on and off the throttle?

Here's a one possible solution to he parts/rebuild issue :
http://www.ericgorr.com/hondacrf290.html
 
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#3
It happened in a whoop or roller section started makin a noise then shut off,leaking oil and coolant out the front of the engine. After i pulled off we checked to see to see if it was lock up and it was.The bottom of the piston was broke off on one side where the wrist pin connects to the rod the other side where the wrist pin goes is still intact.The rod broke appox 2 in from where the wrist pin goes and had a slight twist to it.The cylinder had a few deep scratches that may have been from the piston twisting when the one side broke and a gouge that looks as if the rod may have hit it.The wrist pin was found in the bottom of the motor.The bike is stock no motor mod except for a boysen quick shot.The valves where kibblewhite black diamond.When I did the top end I just cleaned the piston and put new rings on it because it was in spec.Sorry to be so long just trying to be as specfic as possible....do they have stock motors or are they all 290's~~~thanks again for your help
 

Rich Rohrich

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#5
russell.k said:
It happened in a whoop or roller section
The most common cause of this type of rod failure or as Mike Perry from Kibblewhite calls it "energetic disassembly", is connecting rods that break near the small end of the rod . Usually seen as a plastic failure of the rod.

Rod failures often occur when a rider shuts the throttle completely from high rpm. It's more common to see a rider break an engine entering a corner or in a whoop section than it is running down a long straight.

Closing the throttle stops the cylinder from filling the cylinder completely which effectively lowers the cylinder pressure.
It's a lack of pressure that is the problem. If you stop the cylinder from filling there is a lack of pressure above the piston as the piston decelerates approaching TDC. When the cylinder is full and firing the combustion pressure works as a cushion to help slow the piston down. Without that pressure the rod is subjected to a much greater tensile load and often times a plastic failure of the rod near TDC occurs.

The same thing happens when you hit the rev limiter. It's not the rpm that kills the rod and the crank bearings it's a lack of pressure above the piston that is the problem. If you stop the cylinder from firing there is very little pressure above the piston as the piston decelerates approaching TDC and the rod gets stretched like Silly Putty. Just like Silly Putty if you stretch it far enough and fast enough it will break.

This is most often seen when up a new top end is done on a crank that is past it's service limit. If you keep on top of normal parts replacements none of these issues are ever likely to come up.



The point being, if you stop the cylinder from firing at peak revs it puts the crank, rod, main bearings and piston in harms way whether you cut the engine with the ignition or the fuel.

That said it would be possible with a proper engine management system to roll back the ignition timing, or cut fuel in a progressive way to slow the engine down rather than just killing combustion. But, it's important to remember that a large number of engine over-rev failures that I've seen were caused by riders mechanically over-revving the engine by driving it with the rear wheel (downshifting at high rpm, etc) . There is no rev-limiter, no matter how smart it is, that can stop riders from being stupid. The OEMs are in the business of selling parts so a simple rev-limiter might be better for business than a complex DUMBASS-limiter.
 
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#6
Rich thanks for all your info.After reading it a couple of time i see what you are talking about.Can the same thing lead to a piston cracking in half.(what looks like a stress crack all the way across the piston almost to the point of breaking inhalf )That was a seperate incident with our other 05crf250r.It lost power and compression and that is what we found to be the problem with it.thanks again for
 

IndyMX

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#7
I wonder if AJ has any 250F motors laying around still.. That could be a pretty good option..
 
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#8
I wonder if AJ has any 250F motors laying around still.. That could be a pretty good option..
How can I get ahold of him~~~~~~
 

IndyMX

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#9
russell.k said:
I wonder if AJ has any 250F motors laying around still.. That could be a pretty good option..
How can I get ahold of him~~~~~~

Click the Service Honda logo on the left.