piston slap and powervalve rub -pics

[freakystyley]

so I finally got around to diggin into the '95 YZ250 I thought had a bad crank bearing. I found the piston to show what looks like powervalve rub - I posted a few pics in my gallery here http://www.dirtrider.net/gallery/browseimages.php?c=3&userid=73918
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what I though sounded like a bad bearing might have been piston slap - the piston skirt is worn a lot (67.66mm) on lower back side - may have been the source of my noise?
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crank bearing "feels" smooth - slight rub at TDC - but crank is still in the case...rotor/mag and clutch still installed - don't really want to crack it if the top end is to blame. Crank "looks" clean, no burns, no pits, etc. And no debris came from the case after a quick premix flush
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what can you tell if anything from these pics?
(other than I suck at taking close up photos!)

 

[76GMC1500]

I see no power valve rub? The damage is on the intake side. I can only think of lack of lubrication for that.

Was the piston installed backwards in the cylinder?

 

[freakystyley]

I just went out and rechecked - it is the exhaust opening that is chipped and the piston had the arrow facing forward (toward the exhaust opening). the one pic of the piston with the shiny spot between the black marks is what I thought was P.V. rub spot. What damage do you mean, the chip or the piston - you are right, that is the intake side of the piston with the wear - I run 32:1, maybe previous owner ran lean...
would that cause the piston slap? it is .30 below spec.

 

[76GMC1500]

That chip isn't really anything to be concerend with. That piston appears to be OEM, but the chip appears to have been there for a while. Maybe the cylinder just chipped and the piece left through the exhaust port without doing any damage. The .30 below spec will certainly cause slap. What about this rubbing of the crank? It should turn free. If the crank is good, I would replace the piston and keep doing what you're doing. I would expect lack of lube wear to be higher on the skirt, but you never know.

 

[1BAD250]

looks to me like you need to champfer your ports a bit looks like the piston is slightley rubing the ports when its at operateing temp. ive had a lot of people bring me engines blowed or locked up cuz the rings & pistons snaging ports.you can do it your self with a fine round type file dont go crazy just taper them back a bit.