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Rm85 piston has a notch in it?

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Hey guys, I'm normally a 4 stroke guy but bought a 2007 rm85 that needs a new top end. When I removed the jug and was looking at the piston I noticed the piston has a weird notch cut in the ring gap. Is this normal or should I buy a new piston or what? ImageUploadedByThumper Talk1407612247.910365.jpgImageUploadedByThumper Talk1407612259.778446.jpgImageUploadedByThumper Talk1407612269.112211.jpg

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That notch definitely isn't right the ring should have a flush groove all the way around it.That would give multiple problems.Looks like the engine was running hot.Look on the under side of the piston on the bottom of the crown and see if its black.If it is that's an indication of the crown getting really hot.Could be from a few different thing such as jetting, fuel mixture

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Should you buy a new piston?  I have never seen a piston that worn out before.  Look at the clearance between the ring and groove.  The notch was caused by the ring bouncing up and down in the groove every time the piston goes up and down.  I have no idea how that ring is not broken.  I have no idea how the piston ran that long without shattering.  Your cylinder is probably toast, too. 

Edited by 1987CR250R

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Like I said, know little about 2 strokes. And it still would run like this lol crazy right? I went ahead and ordered new piston and rings and the cylinder was fine, took it to a machine shop and they honed it, didn't even look bad (guess I got lucky there). Just wasn't sure what caused this but I guess I know now lol thanks for the replies.

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Make sure the nikasil plating looks good on the cylinder.Also check and set the ring end gap clearance when you put it together.

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I'm new to two strokes so I gotta ask, how do you set the ring end gap clearance? And the cylinder looks good

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Put the ring in the cylinder without the piston then push it in a bit using the piston so its level all the way around then take a filler gauge (what you check valve clearance with) and check the gap between the ends on the ring.You want .004 per inch (theres 25.4mm in an inch) so lets say your running a stock 85 piston which is 48mm close enough to 2 inches so .007 or .008 is good enough. Just take the filler gauge and then make sure .008 fits inbetween the ring gap.if it doesnt hit the ends with a file tell you have the clearance needed.Reason for setting the end gap clearance is that when the piston and rings heat up there's clearance for them to expand and you want just enough to minimize blow by.

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Ok sounds good, I greatly appreciate the good response. I am waiting on parts then I'll hopefully get it runnin.

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