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How to determine if the crank needs replaced.

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Guys,

        I purchased a 2009 RMZ 250 not running.

 

I have attached pictures of the cylinder, piston and crank.  The valves were out of adjustment especially the intake valves.  So I sent the head out to millennium technologies and they are cutting the seats and installing new valves.  I am replacing the cylinder and piston but not sure what to do with the crank.  The play in the crank seems good.  No up and down just some side to side play.  Their is bluing on the crank?  I do not want to put this bike back together just to have it blow up again.  I purchased the low end rebuild kit just in case.

 

I tried to upload a video of the side to side play on the crank but it will not let me upload it.

 

 

Thanks

 

 

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Edited by racerx121

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Bluing is a bad sign, but I don't see any indication or overheating on the rod itself.  Could be from the assembly process.

 

If you have the parts to rebuild it you might as well.  This bottom end is questionable.  Personally, I would run it if the bearing feels good and there is no up and down play.  Its different when its someone elses bike though.

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Your manual should show a procedure and specification for measuring small big end bearing clearance by afixing a dial gauge at right angles to the side of the small end and rocking  the rod on the crank without allowing it to slide along the crank pin as it rocks.  This does not give a direct reading of the actual clearance, but is a calculated value based on th elength of the rod, the width of the bearing, and teh allowable clearance range.

 

An example from a Yamaha manual is attached; see "F"

 

Also, because the piston brought  the engine to a halt, the crank should be removed and checked for being correctly aligned, including the question of whether there is any bend or twist in the rod itself.  You should also know that in cases where the piston stops the engine, it's possible for the force to have hammered an imprint of the rod bearing rollers into the races, which will lead to the bearing failing in a fairly short period of time.

 

The bluing on the crank cheeks means nothing.  It's the result of the factory's use of heat to assemble the crank. If the big end of the rod were blackend, that would be different.

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Bluing there more likely is the result of how they put the crank together, could be they heated to get the fitment . You have to watch out for bluing primarily on the rod ends and bearing surfaces.

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If there's absolutely no up and down play, and it feels perfectly smooth with no rumble or resistance when rotating you should be good to run it for a while longer.

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If there's absolutely no up and down play, ...

 

...It would be seized.  The big end bearing has clearance in it when it's brand new, and wears looser in use.  The fact that you cannot feel or see the play is due to the fact that is only supposed to be about .001".  By the time you could detect it, it would be well out of usable range.  Bad advice.

 

In the clip from the manual, notice that "F" is measuring the degree to which the rod can rock or tilt side to side WITHOUT allowing it to slide side to side on the crank pin.  So, what is being measured is not big end side clearance (which is "D"), it is the left-right "swing" of the rod, which is directly due to the clearance at the big end.  The numbers used as tolerance are so large because they are exaggerated by the length of the rod as opposed to the width of the bearing, a fact that is factored in.

 

The simple fact that you can't make it rattle in place doesn't mean it's not worn to the limit, and you won't know unless you measure it the right way.

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Anytime one gernades like that, I'd go ahead and put a crank in it.

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No up and down that you can feel is what I meant.   If I feel any up and down, then I replace, if not I run it a while. That method has worked for me for 40 yrs.  If that was my bike I would definitely split the cases to inspect further, though.  You will likely find debris in the bottom end.

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I replaced the crank just to be safe.  There was a some side to side slop that bothered me.  I didn't want to risk it.

 

Thanks guys.   Its up and running.  Trying to figure out the jetting.  I went down on the pilot and one up on the main jet.

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