Piston Ring

Bernoff

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Hey guys i have a 2015 450 SX-F KTM. In manual it says the gap should be 1.mm. I am assuming that is the max but what is the min gap allowed?
 

Legend14

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Minimum shouldn’t matter much as long as they don’t touch when installed.
 

Bernoff

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Ya I have to agree with ABMax24 on the need for min gap specs. because one guys don't touch spec and another guys don't touch spec are totally different. I have mine apart and instead of 1.mm it was .330mm. Talked to some guys and they agreed it was getting close. One really hot run could spell a wreck. Also have to say that Legend 14 is not the only one to tell me long as they don't touch your good to go. I,m not a licenced mechanic so finding the don't touch method hard to wrap my brain around. I knew 2 strokes were fairly forgiving when working on them but thought 4s were totally different ball game.
 
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ABMax24

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As cylinders and piston rings wear the ring gap increases, if it's a used set of rings that are going right back in the cylinder they came from then I wouldn't worry about it as long as it doesn't exceed the maximum. If your going to squeak a ring it'll usually after a rebuild with new rings and not checking the gap. That being said .33mm is tight, that's like 0.013".
 

Bernoff

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Would have been interesting to know what gap I had initially. At this point I am not going to put the old parts back in. I was hoping someone else with a KTM could tell me where to look to find more reliable specs. Don't want to be on the hill and end up squeaking it. I figure if a guy gives something to much gap its pretty obvious your going have pretty poor performance. Minimums are more important to me. Still like to have info on both ends of the specs. Maybe more schooling is in order for me first. Lol.
 

ABMax24

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Would have been interesting to know what gap I had initially. At this point I am not going to put the old parts back in. I was hoping someone else with a KTM could tell me where to look to find more reliable specs. Don't want to be on the hill and end up squeaking it. I figure if a guy gives something to much gap its pretty obvious your going have pretty poor performance. Minimums are more important to me. Still like to have info on both ends of the specs. Maybe more schooling is in order for me first. Lol.

Wiseco has a good yet lengthy article on piston ring gaps, have a read through that.

I've always been a believer that a couple hp lost is better than a seized piston, as such I usually gap my rings a couple thou more than spec, particularly on my boosted 2-stroke.
 

geelak900

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What brand of piston are you using? Usually the piston manufacturer specifies their min. gap. Example, CP piston in a Thumper Racing big bore kit specify .0045" per inch of bore size so min. gap is .018" on compression ring. Oil scrapers are min. .005 per inch of bore.
 

Bernoff

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Haven't decided yet. Recomendations? Honing my computer geek skills right now.
 

geelak900

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You just looking to freshen your 450? I have used ProX with no issues, price is good. OEM pistons are good but pricey. Wiseco and Wossner work fine too. If you really want to wake that snowbike up put a 520 kit and an ECU on it. You will pull another gear everywhere with it and then some. Let me know if you have any interest, I have a kit in stock for that bike.
 

Bernoff

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Any of you guys do a LIGHT hone on the nykasil before reinstall?
 
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