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Discussion Starter #1
Any thoughts on why the second ring is pinched? It's obvious that it touched/scrubbed the cylinder wall, just don't know why. Have never hurt a plug on this cyl, and frankly, I can't ever remember having any heat in this plug, ever. #4 on a bbc.

 

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The nitrous guy
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I am sure that is at the 12 oclock position on the piston. The sharp edge at the top of the piston tells you that it is not likely over timed as no alum is moved around. The skirt is scuffed, but not beat all to hell, so WAY too much clearance is not an issue. I KNOW you have already heard this........lol......but it is likely just too much fuel. This does NOT happen on one pass but over time. Extra fuel can lead to extra heat in the rings. We know rings have two jobs. One is to seal the cylinder, the other is help dissipate heat from the piston into the cylinder wall and water jackets. If the rings are ALREADY hot, they can't pull heat from the piston as well. The weakest point of any piston is the ring lands. They get hot, they are thin and easily move around. This is how a second gets pinched and a top opened up. The ring is just a scraper and on the down stroke is trying to pull the top off the piston and on the up stroke is trying to push everything off the bottom of the skirt. If the lands are overheated, the friction from the rings can physically cause the land to move and wahla.........we have pinched second and lifted top.

The shiny spots you see are likely where the piston swelled, or moved, however you want to look at it, once the lands got hot and then touched off on the bore, You bend a machined part straight down, the top of it now sticks out further that it did and closes the gap between it and cylinder

Monte
 

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My BBC setup did this several times. I didn't have enough quench increased head gasket thickness and never had an issue
 

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Discussion Starter #5
Thanks Monte. Whatever it was did happen over time. When the motor was new, all cylinders compression tested within a few psi of each other. The last time I compression tested it a couple months ago, it was obvious that #4 was lower than the rest. This time, it was way low (low enough to not run again).

Thanks for your input Bobby. The quench is certainly tight on this engine, as it is zero decked and has a .051 gasket. When piston rock comes into play, and this being the passenger bank (where quench gets tighter), I'm sure quench gets down into the .030 range.
 

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Discussion Starter #7
Was the gap in the oil ring in that spot when it ran?
I think it was...I didn't pay that much attention, and I didn't purposely move it either.

I pulled #2 out to see how things looked there. The rings on #2 had noticably more tension than on #4, so heat was definately an issue for #4. Gaps were .044/.044...wider than what I would have expected. When I bought this motor, it was brand new and together already.
 

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That's a real nice shot! Aluminum fatigues If you look at the top ring it doesn't look very concentric either. I believe ring clearance have a lot to do with it also. No manufacture are perfect. I pinch some rings once, knock the rings out that were stuck in the pistons took a hand filer , slid the filer inside the piston ring groove. Never pinched those rings and pistons again lol.


Not recommending it that's what I did. That motor has been kicking ass and taking name with them same pinched pistons.
 
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