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I am hoping that someone can answer this. If the skirt of a piston has been rubbing on the cylinder, does it make the piston garbage? When I disassemble my engine for an overhaul, I noticed the pistons were worn on the skirts. There was no dissernable problems before, besides a small knock which is what prompted the rebuild (plus it has 70000kms on the clock). When I measure the diameter of the pistons, they are still larger where it is worn than other spots. Since rings do all of the sealing, what are the drwbacks to using a piston like this? This is a pretty technical question, but hopefully the answer will save me some grief, either now or later.
 

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Measure the piston at the bottom of the skirts (from front to back/ 90 degrees from the pin) and see if it's in tolerance. Where is the knock located, top or bottom end? If it's the top end, it could be piston slap/loose cam sprocket/cam end play.
 

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What do you call wear? if the pistons show deep grooves on the sides like they have been shot across the asphault, you need new ones. I would like to see a picture of these pistons. my rule of thumb on "worn" areas, was if the rough patch is deep enough to catch a finger nail on the trash the piston.
 

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I'd be curious to know the condition of the sleeves. Be sure to measure them carefully top to bottom and around. Compare to specs. At 70,000km. on the engine I'd junk the pistons for sure. The bottom end may need attention with those k's. Especially since you had a knock. If you're going to do it, you may as well do it right.
 

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When I disassemble my engine for an overhaul, I noticed the pistons were worn on the skirts.
If you dont replace the pistons you risk another disassembly for another overhaul.

The manual has specifications for piston measurements, they should be followed.

I do not really understand an engine disassembly for overhaul then not replacing parts that are questionable, why take it apart?

The engine knock might be because the piston skirt is rattling in the cylinder, however if that is happening it would sound more like a knock with ringing or pinging, just a knock.

Personal opinion, I would replace any under spec or close to spec parts unless I was planning getting rid of the bike. If you are keeping it fix it completely, it will pay you back with reliability.
 
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