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Discussion Starter #1
Had some problems with the car at the track the other night. Just got around to pulling it out and diagnosing the problem.

For starters the abbreviated problem. Brought the car to the line with a 102n/91f nitrous tune. Mashed the gas and received a big backfire in return so I went easy down the track. Got the car back to the pits and I noticed a ton of blowby, after I put out a fire on my vaccume catch can! So on the trailer it went and to the bar I go. The car has been acting up at the track this year so I decided to yard the engine out.

Once I get it out I start a compression check, second hole I check is #6 and it is DOA so I stop there and pull the top off.

Here it is with the top end off. Number 6 is at the top of the hole. You can see a little skirt scuffing on #4. Cross hatching still visible and piston tops show no evidence of serious detonation. More importantly, the ring lands are in place.
https://photos.app.goo.gl/FpZHLfqedyHY9dtG9

Here is 1-7, look equally good but I wonder if there is a little water getting into 5?
https://photos.app.goo.gl/entweAg9t62yHTr67

So back to #6 where I know I have a problem. With is down the hole you can see the skirt is for sure scuffing the wall. but again more good news there is no major carnage which would require scraping the block, DART SHP.
https://photos.app.goo.gl/hp9RGRo2no8F7t3A6


And here is the problem. Top Ring is stuck. I'm no expert and I really don't know the official term for this condition but I do know enough to know it ain't good. The top ring will move but not a lot, I can probably peel it out of there but I thought I'd leave it for my builder. You can also see the piston skirt is scuffed up pretty good.
https://photos.app.goo.gl/e9u9wNtcWHfq5uDh6

Another shot of #6
https://photos.app.goo.gl/Qx3MAvBdYAnVVKk99


So while I'm here this is #5 and you can see the cyl wall is lightly scuffed and the cross hatching is still there too.
https://photos.app.goo.gl/vLiX3gxDfbxv4HZa8


So what is the official term for a stuck top ring? Also why are my piston skirts getting into the walls?

Engine WAS a SBC 421 with a 3.875 stroke, 4.155 bore, and a 6" rod. I'm guessing I will be looking at pistons so a set of .165 or .185's maybe in my future. Probably freshen and work the AFR 220 heads too and look into a SR cam. First world problems....
 

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My guess is the excessive pressure caused the ring land to pinch the ring. So the top of the piston is compressed. But basically requiring pistons in those holes.
 

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Wonder if it was heat. My AFR's are pretty rich so I wonder if it could be one lean run?


Also I lost my vacuum plug on the carb a while ago, could that have created a heat problem too?
Lean is mean....a big vacuum leak and nitrous are not friends. If it popped on you....it was lean..
 

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Discussion Starter #5
It had to be that leak and a lean condition that heated the combustion chamber. I normally see fat AFR's when spraying. I have tried to lean it out but it never runs right. I also pull a bunch of timing and run a cold plug and they look really good after my runs. The piston tops tell me I'm not too fat and have the timing right as well.


Oh well, It always comes down to a 10 cent part and/or stuffing big hands in small places. I think I knocked the vacuum plug off/loose while changing the rear jets. It has a zip tie on it now and will be checked/replaced yearly as I am now the owner of 12 original Holley replacements.
 
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