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Yz 250 Piston/Cylinder Diagnosis


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How's it going fellow yz riders?

 

As many of us do, I began my annual winter inspection last night and finished when I got the cylinder off the bike.  Upon further inspection today, I'm finding some signs I'm not too happy with  :banghead:

 

Bear with me here, I'm not very experienced with engine rebuilds so I'm sorry if some things here sound obvious.

 

Looking at the piston (approx 40 hrs), there is some blow by on the exhaust side, some carbon build up on the crown, and abrasion on the exhaust and intake side of the skirt:

 

Exhaust Sidehttp://s1093.photobucket.com/user/chris8811/media/Piston1.jpg.html

 

Intake Sidehttp://s1093.photobucket.com/user/chris8811/media/piston2.jpg.html

 

"Left Side":  http://s1093.photobucket.com/user/chris8811/media/piston2.jpg.html

 

"Right Side":  http://s1093.photobucket.com/user/chris8811/media/piston5.jpg.html

 

Crown:   http://s1093.photobucket.com/user/chris8811/media/piston4.jpg.html

 

 

To me, the blow by seems ok (considering the hours), but what I'm worried about is the abrasion on the sides.  This looks like the piston has been banging around in the bore.  Upon further inspection this appears to be the case.  I washed the cylinder with WD-40 and some scotch brite pads and found two areas of concern:  The first was that at the very bottom of the cylinder, it appears that the Nikasil was worn away.  This appears to be from the intake side hole in the piston hitting the cylinder in that spot.  The second is that at BDC in the cylinder, there are rings in the cylinder.  This spot's bore (and only this spot) is out of spec( about .01mm too large):

 

Worn Nikasil at bottom of cylinder:    http://s1093.photobucket.com/user/chris8811/media/cylinder1.jpg.html

 

Ringed wear in cylinder:  http://s1093.photobucket.com/user/chris8811/media/cylinder6.jpg.html

 

Another angle(note the wear below the power valve):  http://s1093.photobucket.com/user/chris8811/media/cylinder5.jpg.html?filters[user]=121351835&filters[recent]=1&sort=1&o=20

 

Bore Measuring Method (Awesome tool!):  http://s1093.photobucket.com/user/chris8811/media/Borematic.jpg.html?filters[user]=121351835&filters[recent]=1&sort=1&o=10

 

 

What do you guys think?   My initial Idea was to do a top end this winter but it appears that I'll have to get a new cylinder?

 

Any feedback/advice is appreciated.  I'm a bit lost as to what to do as I've never had to replace a cylinder before.  

 

 

Also, I'll try to get the pictures to show up instead of posting links.  TT isn't being very cooperative with me tonight..

 

 

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No need to replace cylinder, get a new piston and send it with the barrel to someone who does nicasiling so they can do the barrel to match the piston.   

 

What I meant was that I would do the cylinder core exchange with some company. For some reason I was under the assumption that this was cheaper than a replate.  Looks like that is not the case.

 

 

 

Looks to me like you're not warming the bike up enough...

Piston crown looks good, good wash on it...

Cyclinder pictures are tough to tell anything cause they're not in focus...

 

You know I said that to myself at first.  It is suprising too because I usually wait until the radiators are hot to the touch before I pull away.  Maybe I need to take the first 10 minutes a little easier.

 

I'll try and get some better pictures after work, uploading them to the computer degraded the quality of the pictures quite a bit..  Basically what I'm seeing is that right below the ports (above them in "Ringed Wear in Cylinder") there are two rings worn into the cylinder.  They are right where the rings would be at BDC.  When I was measuring the bore, every other location in the cylinder measured 66.403 mm - 66.405 mm.  At this spot, the bore measured 66.416 mm.  I'm just not sure why this spot in particular would wear so much more than the rest.  But as I said, I'm no engine expert... 

 

 

Thanks again guys, hope to have those pictures up soon.

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What I meant was that I would do the cylinder core exchange with some company. For some reason I was under the assumption that this was cheaper than a replate. Looks like that is not the case.

You know I said that to myself at first. It is suprising too because I usually wait until the radiators are hot to the touch before I pull away. Maybe I need to take the first 10 minutes a little easier.

I'll try and get some better pictures after work, uploading them to the computer degraded the quality of the pictures quite a bit.. Basically what I'm seeing is that right below the ports (above them in "Ringed Wear in Cylinder") there are two rings worn into the cylinder. They are right where the rings would be at BDC. When I was measuring the bore, every other location in the cylinder measured 66.403 mm - 66.405 mm. At this spot, the bore measured 66.416 mm. I'm just not sure why this spot in particular would wear so much more than the rest. But as I said, I'm no engine expert...

Thanks again guys, hope to have those pictures up soon.

The stock nikisal is quite weak on YZs. Not that it could be more wear, could just be thin in that area and it's worn through...

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Warning: Pictures are huge, sorry about that.  I'll make sure to resize next time.

 

Anyway, Here are the rings I'm talking about.  Look right next to the ports  

 

 

20141223_222808.jpg

 

And here is the wear spot I mentioned:

 

20141223_222852.jpg

 

 

 

 

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Hone it, measure it get appropriate sized piston. I noticed a big difference with that scuffing look when I went to Mogul 800. The black coating stayed on piston twice as long. Telling me less wear.

 

I imagine you mean one of the ball hones correct?  Interestingly, I have been using Motul 800 for the life of this piston.

 

 

Blow by....you ain't seen blow by till you look at this one I bought and rebuilt. Kustom Kraft just did this cylinder.

 

Oh trust me, I wasn't complaining about the amount of blow by.  I was actually quite happy with the way that looked.

 

One time I worked on a friends cr125.  The piston skirt was so dark and stained I couldn't believe the pos was still running.  The clearances were so far off it was a miracle this thing ran as well as it did.  

 

In fairness he did get it for cheap...

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It doesn't look that bad, really.  Hard to tell if the plating is coming off from the pictures.  It will look like an old bumper that has the chrome pealing off when it happens.  To me, the Buffing on the skirt could be from worn rings or wrist pin cocking the piston and touching the cylinder a bit.

 

As for the rings on the bottom??  I don't know what you are describing, really.  Do you mean the little scratches that kinda over lap in XXXXX?  That is what you want all over the cylinder when fresh.

 

Take it to a shop and get the ideas.  From here in my comfy chair, I would see if a ball hone and about 2 min or a little honing could do anything.  Might go back with a slight oversize and fresh rings and go through a very slow break in.  

 

If you use a ball hone, not sure if you have done it before.  Go slow, lots of light oil for lube and keep it cool, move the drill up and down while it turns at about 500 rpms.  You will see that XXXXX  show up again.  Don't do anymore than you have to.  Remember you are taking material out of the bore.

 

The next option is the replate and piston/rings.  I know it costs more, but I would get the piston from the plate co.  That way they can get the correct piston, and fit it perfectly.

 

Patience and ask questions!!  You have all winter!

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