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Nicked my oil ring

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When putting my motor back together I caught the small oil ring at the bottom of the piston on the bore, it's back together now and running but paranoid of it damaging the bore. Do you think I should pull it back down and replace the ring? Or should it be right if it's running fine? Cheers

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The oil rings are easily hard enough to do significant damage to the cylinder, in spite of the fact that Nikasil is harder than the edge of the oil ring.  The severity of the twist that the oil rail got is the key to whether you can get by with this, and I can't recommend that anyone ever try to, but I've seen it done before, with and without problems.

 

But it didn't gouge out a ditch in the cylinder or lock the engine up in the first two minutes, so you dodged that possibility.  And it's not smoking.  Yet.  Are you in the clear?  There's a fairly good chance that you are, but you could still have trouble later if the ring puts a pressure point on the bore wall.  Time or a tear down will tell.

 

The top two rings seal combustion gasses in the combustion chamber, always imperfectly with conventional rings, because of the gap at the ends.  The very small percentage that leaks is "blowby".  

 

The bottom ring is a composite assembly consisting on two scraper rails and a corrugated expander.  Their purpose is to keep oil out of the combustion chamber.  The ring groove this set sits in is slotted so that oil scraped down off the bore wall by the upper rail will have somewhere to go besides piling up on the lower rail.  The slot also serves to lube the upper rail through splash from the crankcase.  It's a mechanical irony that the oil rings won't work very well unless they have oil on them.  They wouldn't last very long without it, either.

 

To understand how well they're required to work, consider that an average 5000 RPM, burning 100cc/hour means there is .00066cc in the combustion chamber each time it fires.  Doesn't take a big hole for something like that to get through.

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Some good information there cheers, it was the top ring of the oil scraper you explained, no smoke yet il see how it goes fingers crossed I've dodged a bullet.

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