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Evansaw

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Got 3 identical OEM Stihl661 pistons and believe they are made from Kolbenschmitt

They've seen just a few hours use, really low

My question is: Why all of them polish on the lower intake skirt? You can also see the exhaust side as new.IMG_8144.jpegIMG_8145.jpeg
 

srcarr52

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This is common on wide intake ports. The pistons have too much out of round (oval) and the intake skirt is getting drug into the intake floor instead of being supported by the narrow strips of piston contacting the bore on either side of the port.

It also happens a lot on saws that ingest a lot of fine sawdust.
 

Evansaw

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This is common on wide intake ports. The pistons have too much out of round (oval) and the intake skirt is getting drug into the intake floor instead of being supported by the narrow strips of piston contacting the bore on either side of the port.

It also happens a lot on saws that ingest a lot of fine sawdust.
So this should be considered as normal as we all normally have NORMAL PORTED saws with wider intakes
 

srcarr52

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So this should be considered as normal as we all normally have NORMAL PORTED saws with wider intakes

It's worse on some model saws. Short piston heights are less effective at controlling the piston rock, offset pin bores towards the intake side create more skirt load on the bottom of the intake skirt.
 

Evansaw

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It's worse on some model saws. Short piston heights are less effective at controlling the piston rock, offset pin bores towards the intake side create more skirt load on the bottom of the intake skirt.
Now that's serious
Hopefully on my ported 661s the subject should be just the wider intake.
At my maxed out one I went to the heavier but wider skirt piston of Meteor.
Not sure how much HP performance lost or torque added. At Stihl mgd1 the saw went at 13.700+rpm though
 

drf256

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It’s the thrust side of the piston. As it pushes down, and the crank is pushed forward, the rod pushes the piston back against the intake wall.

That’s why debris, water, large intake ports accelerate wear. All saws will wear the intake skirt >> exhaust skirt for this reason.

Perfectly normal. Just make sure there isn’t more then .002-.003 clearance between the intake skirt and the bore if you’re reusing the slug.
 

Evansaw

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Gods of porting comments gave me piece by piece a huge expansion at my knowledge

Feel lucky to be surrounded by you

Thank you gentlemen
 
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