Uh Oh piston damaged.

Ahh, the joys of 2nd-owner motos. I hear you... Why is Old Mate previous owner always a total dunce when it comes to maintenance, putting oil & water in the right spots and replacing sprockets? Not to mention keeping an air filter clean and oiled.

Sorry, can't offer any help about your piston dents but I like your hunch about the non-spec spark plug, and thanks for the tip using the Soob cleaner for the KIPS. Last rebuild I did started (too many) months ago while I was living in a tiny, brand new rental apartment and scrubbing the KIPS without ruining the white laundry walls was quite an effort. Anything to make that little job quicker, cleaner and simpler is great!

Is the seat where the plug screws in badly worn? Or did someone run a plug without the washer on it? I'm not sure how much tolerance there is inside the head when it's at TDC - but maybe they could cause a plug to go too deep? It'd be worrying if there was something ELSE hard enough to do that in your cylinder! Good luck with the rebuild.

You might want to check if your main bearings still have the balls in them... they might have evacuated into the cylinder and bounced dents in. Man, that would make a wicked pea-shooter out the pipe...

Jokes. Useless jokes.

Depends on the 'dents.'

Such impressions can be left over from the bits of metal (melted for a number of possible reasons) that are not attached to the piston crown anymore.

Have a big carbon buildup on the bottom of the crown? That's a sign of excessive heat on the top.

Excessive heat can come from:
Spark Timing (too advanced)
Air/Fuel mixture too lean
Failed/failing cooling system
Wrong heat range plug (you will find argument on that part. NGK says a one step heat range change of plug changes the combustion chamber temp +/-50º I don't know I buy 100% of that).

Dents can come from FOD (foreign object damage). Something ingested that shouldn't-a been.

Detonation can destroy an engine. That can have bits and pieces flailing around in the combustion chamber after they fall off of whatever they fell off of. I could show you a piston with a whole bunch of ring land missing. The top of the piston (what was still there) looked like it had been beaten with a small hammer.

If your 'dents' are a depression right under the spark plug, it's likely molten aluminum missing from the crown.

Parts of spark plug can break off. Are you using a 'P' type (projected tip) plug? Don't do that!!

I could show you a piston with a cool intake (unfortunately, also an exhaust..) vent. That came from a spark plug disintegration. It was a fine wire that had been in the bike too long.

What is your big-end rod radial play? That would be bad if it was that sloppy.

Before you put it back together I'd suggest finding out what is wrong in the first place.

Correct plug?
Too much 'shave' off the head (cc it)?
Cylinder spec correct (not shaved there, too?)
Rod measurments
A/F too lean (crank seal leakage? Intake boot crack?)?

etc etc

Basically everything..

re:
'100% caused by the plug strap'

I don't know what that means. IF you mean the ground strap is what's banging on the piston then you have some serious clearance problems.

Yeah...that was brilliant, 'eh? :clue:

I second checking the big end of the rod. I HAD a 94 cr125. I opened it up and found the same thing. I put a piston in it and put it back together. Several days later the large end bearing went out and the rest is history.

You betchya you gotta split the cases to change the big end. "While you're down there..." why not rebuild everything? It will make you feel good.

lankytim's tip: If you go ahead with replacing the main bearings (pretty cheap, about AUD$45 for two) I can give you the advice to most definitely buy genuine Kawasaki parts there... I just did a rebuild on my Katie and there was a problem when I put my cases back together - crank very tight to turn. (Much swearing and annoyedness.) Took it to the Kawa shop and they split it and couldn't find a problem, crank was pressed back together in spec, etc. I asked them to put new, new main bearings in it. They couldn't get genuine Kawa parts in in time for the main bearings so asked me if I'd accept aftermarket parts - sure, the numbers match up and I had originally put same parts in. Wrong! They discovered that although the bearings are otherwise identical the Kawasaki parts have a bevelled inner race that fits snug against the inner end of the crankshaft against the weighty-bit, ie. makes the whole assembly slightly thinner and subsequently less tight when the cases are back together.

The most insulting part is that I paid about $4 extra for the non-genuine parts in the beginning...

Probably nothing new to the more experienced rebuilders out there, but just thought it's an important tip if you go ahead with the rebuild.

New rod and big end kit, plus labour, was around AUD$180 at the Kawasaki shop. Not one to attempt at home unless you have a pretty schmick workshop.

Can't offer much help on the diagnosis, but I suggest that the plug is not what caused the dents. If someone used an incorrect (longer) plug, you might have a 'dent', but not plural. The piston is going to come up to the same spot each time, hence one impact point.

Also, after the first contact the ground strap would likely crush against the center electrode, closing the gap.

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Those aren't from no sparkplug....even a splitfire! ;)

re: covered in carbon?

Not recent then.

I'm still on the FOD track......The piston is considerably softer than the head.