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crank condition

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I recently picked up a 2006 yz250f. The guy I bought it from said it had been to the dealer, and they told him the motor was toast. Took the head off it looks good, cylinder is clean, no grooves or marks at all, so I figure bottom must be gone. The crank turns smooth, no sound from the bearings. Rod has no vertical play, but side play is about 2-3mm. The only thing that looks odd is a burnt looking mark on top of the crank, on either side of the rod. What are the signs of a bad crank? Is 2-3mm side play out of tolerance?

Thanks

Duncan

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Sounds like the crank is toast. Several signs of it. Tests I use: 1. Is the wrist pin loose in the small end or in the piston, or are there groves in the pin? 2. Look at the teeth on the crank. Are they sharp and pointed? 3. Have you split the cases yet? Did the sprocket come past the bearing or did you have to grind them down to get the crank out? (Manditory replacement) 4. Does the rod have side-to-side play? (You already said it did) They usually don't, though they will move slightly. It doesn't use spacers like an XR250 crank does. 5. Are the sides of the flyweights turned blue from getting hot where the rod was rubbing on it? I saw one failed crank (kid ran it on the rev limiter) that rod was welded to both crank halfs.

I'd say from the way you describe it, it was knocking pretty good and the dealer said $1,100 to fix it. You can do a crank, top end, and replace the valves if you do all the work except cut the valve seats for about 800 or so and have a new motor that will run great.....

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Bluing on the crank near the big end is normal. Heat is used during manufacture.

'Toast' means little. It could mean it had been raced and is due for a freshen, it could mean it burns oil. It could mean the valves are receding and need to be done.

But based on the info supplied to us, it is all pure conjecture.

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side to side play for the small end of the rod is normal, but that seems like too much. Check a manual to find out the specs

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Bluing on the crank near the big end is normal. Heat is used during manufacture.

+1

I've seen this on every YZ i've worked on.

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Post a picture Then someone could give you a better diagnosis.

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Thanks for all your replies. It sounds like the marks on the crank halves are normal. I am going to measure the small end play, but I'm sure it is over the 2mm that the manual states. I am going to replace it with a Hot Rods crank.

Thanks

Duncan

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