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Discussion Starter #1
Historically I have seen many broken ring lands here on this board from both the 4a and the 3S crowd. This has puzzled me.
The blame has been that detonation caused it.

But maybe I have found out another reason...

What if your top ring end gap is too close?

so you set your ring gaps up to the Toyota specs. Now you up the boost to humungo levels. Now boost equals heat. Power equals heat. The top of that piston is getting friggen hotter than my wife after she found out I bought another MR2!

That top ring is super heated and can expand to the point where the ends touch. Now they have no where to go. as the ring breaks it can also cause the ring land to break.

Your feedback to this is welcome and encouraged
 

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i may be completely wrong about all this, but from what i can remember from physics, thermal expansion should not cause the gap to close.
 

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What would have to be checked is if the increase in circumference of the aluminum piston due to thermal expansion is greater than the increase in circumference of the piston ring due to thermal expansion. Since the thermal expansion of aluminum is much greater than that of steel (what is the top ring made of...?) I'm guessing the gaps should increase with heat.
 

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Bill, I think you are 99% there. You say that running the high boost creates high heat, the top of the piston gets hot. That right there is pre-ignition and we all know that ridges are hot spots for pistons. That leaves the whole outer ring land as a heat sink at the corners and an optimum place for detonation. I doubt the piston actually touches the cylinder wall, but thats just my opinion.
 

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Discussion Starter #5
quoted from Speedway Motors - Americas oldest speed shop Catalog

ref: Chevy V8 - 4" bore
The Keith Black pistons unique thermal conductivity, ring location and varied end use requires special attention be paid to top ring end gap. KB pistons make more HP be reflecting heat energy back into the combustion process and as a result, the top ring runs hotter and requires additional end clearance. Increasing ring end gap does not effect performance or oil control because normal end gaps are realized at operating temporitures. Failure to provide sufficiant top ring end gap will cause a portion of the top ring land to break as the ring ends butt and lock tight in the cylinder. The broken piece may cause further piston or engine damage.

Dyno testing and track testing has shown that excessive spark advance, lean fuel, too much compression for the fuel and cam used will make heat sufficiant to butt piston rings with as much as .060" ring end gap. The entire top land can expand enough to contact the cylinder walls, when close to meltdown temps are reached.
 

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Perhaps Keith Black pistons are of a design where ring gap decreases with heat, which is not normal...?
 

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i'm guessing they mean that it can expand enough to contact the cylinder wall, and further expansion would cause the gap to close. without the surrounding wall restricting the expansion, the gap should not close on its own. consider a solid disc with a hole in the middle. as it heats up and expands, the hole in the middle will expand as well - it will not get smaller. a good rule of thumb is to pretend that the hole (or gap, in the ring's case) is actually full of material, and expands as well as the rest of it.

this doesn't exactly apply, of course, if the ring expands enough to touch the cylinder wall, in which case it can force the gap closed. i'm not sure if that's what you were talking about, or just about thermal expansion alone causing the gap to decrease.
 

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Bill, what you're saying makes sense. You could probably figure it out if you knew the thermal characteristics of the ring, or knew what the gap was engineered to be under normal temps, and what those temps are (i.e. what the gap is at an operating temp of X degrees vs. what it is when you install it in a cold block). Then figure out what temps are created under high boost and extrapolate out.

I'd also try and think what the results would look like if a seized ring caused the land to break vs. detonation. Would the cylinder walls be scored all the way around vs just in one spot since the ring was putting pressure on them everywhere? Would the land break in a different way if it was being pushed on by the entire over-expanded ring rather than in just one spot where the detonation might have occurred? Would the land even break if the ring seized and stopped the engine--would a seized ring push with even enough force on the land to not let it break, but force something else to go?

Somewhere someone has to have posted what happens to an engine when the ring gap is too small. Somewhere else someone has killed a land due to detonation in an engine that had rings gapped enough to have never touched. I'd just search for each and hope to find photos of the aftermath to compare.
 

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If you know what the physical properties of the metals that compose the rings, you could easily figure out at what temp the gap will close. By using the equation for linear thermal expansion to figure it out how far it would travel. This is the same priciple that makes the thermostat in you house work.

I would write the equation out for it, but There are many greek symbols that the fonts do not have.....
 
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