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Discussion Starter #1 (Edited)
What follow is the summary of what I learned about piston ring’s operations and the role of their end-gap, while building my EcoBoost engine at home:

One very important step while building an engine with aftermarket pistons is to set the piston ring’s end-gap.

Most aftermarket pistons come with rings that have a very small gap out of the box, that must be adjusted in-situ to match the application.

First, why do we need a gap?

There are two reasons for the end-gap: one is mechanical and the other has to do with sealing, but not in the way one may think.

The first reason is simple: if you unwind a 87.5mm 2.3L EcoBoost piston ring you end up with a thin 275mm (10.8”) rod of metal. If you heat it up, it becomes considerably longer.

When trapped in the bore around a piston, the ends might come in contact with each other, after which further expansion has nowhere to go.

The ring’s diameter then increases as it heats up further, scraping the oil film on the cylinder bore, resulting in direct metal contact that instantly creates more heat and more dilatation, rapidly escalating to the seizure of the ring causing irreversible damages to the bore, potentially leading to a piston seizure and the complete the loss of the engine, as the rods bend and exit the block sideways. The piston will seize because the first compression ring is the major path through which the piston crown cools down. If a seized ring adds heat by itself instead of evacuating it, the piston temperature will rise rapidly and cause a piston seizure.

In mild cases the bore damages and uneven ring wear gets you a ton of blow-by and oil consumption over time, and it gets worse every time the engine is loaded to the max as the problem never disappear.

The bore also expands when heating up, and this would widen the gap if the ring itself was not also expanding. It happens the ring expands more than the bore, and thus the end-gap must be set so at maximum expansion of both the bore and the ring, there is still a gap so the ring does not come in contact with itself at the ends, and force itself agains the bore all around.

So far so good, this part is easy to visualize. All it would take is to try a few gaps, different on each piston, on a couple of sacrificial engines to find the ideal gap that is just before ring-end contact at sustained maximum load and be done with it.

Yes, but that’s not all of it.

At first it’s easy to convince oneself that a small gap is good. The ring is a pressure seal after all, and any gap is a leak, right?

This would be overlooking how a ring seals. At first it appears - the ring being of a quite larger diameter than the bore at rest - than the radial pressure that pushes against the bore to create the seal comes from the “spring effect” of the ring itself, however that is not the case. Modern engines have thin rings (1.0 to 1.2mm) and very little radial pressure to reduce friction. The literature says that as little as 10% of the sealing force comes from the spring effect of the rings.

What creates the force that pushes the ring agains the bore is the gas pressure, that find its way behind the rings and forces them to expand towards the bore.

Some pistons have tiny holes in the crown that let the compression and combustion pressures directly reach the back of the ring groove to push the ring out. More commonly, the crevice area (the gap between the piston crown and the bore, upward of the first compression ring) is deliberately larger to enable the pressure to freely reach the ring.

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Some of that pressure finds its way to the back of the ring through the tiny gap between the ring’s top and the top of the groove, but the bulk of that pressure goes behind the ring through - you have guessed it by now - the ring’s end-gap, and paradoxically the wider the gap, the faster the pressure builds up behind the ring, the quicker and harder the ring is pushed out, and the better the seal.

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Ricardo says that “a ring can function only when the gas pressure behind it is is equal or very nearly equal to that above it” and continues saying that “unless this condition be fulfilled, the ring will collapse inward and cease to function as a gas seal. It is essential therefore that the gases above shall have free access to the back of the ring.” Ricardo also says “the initial spring tension of the ring is not very important, since the radial pressure is exerted for the most part by gas pressure.

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Another important aspect is the contact area between the piston and the ring, underneath the ring and against the bottom of the groove, plays a major role in the sealing. Ricardo says “A piston ring seals against gas pressure (1) by its radial bearing against the wall of the liner (2) by its bearing against the lower face of the ring groove. Both are vitally important, but the importance of the second is often overlooked.

From the above, one can see that not only we need a sufficient gap to pressurize the rear of the ring quickly, but we must also ensure the ring stays seated firmly at the bottom of its groove, which is not given.

Too small a gap will slow down the pressure rise and thus compromise the sealing, which promotes blow-by, which in turn results in far more losses 360° around the ring than the pressure loss incurred by the gap itself!

Then another subtle thing happens: when the piston goes up, the ring is seated at the bottom of its groove as the piston pushes it up, and the rising compression pressure (and oil drag) pushes it down.

The pressure also builds up behind the ring and the radial pressure against the bore increases (this is why the bore wear out more near the top. This has nothing to do with surfing the oil film and not having enough speed to stay afloat near TDC, or it would also happen at BDC where the piston also stops, but it does not... the compression pressure and later the much higher combustion pressure expands the top ring and cause the « funnel » wear on the bores.)

So far so good but there is a certain engine speed where the ring just won’t stop when the piston that pushes it up stops at TDC. The inertia trumps the other forces at play and overcomes the compression pressure (and the nascent combustion pressure) and causes the ring to float for an instant, leaking whatever pressure was built behind it and closing the thin gap at the top of the groove, delaying the pressure buildup and compromising sealing altogether.

This is often referenced as ring flutter as the rings float just like valves do past a certain engine speed, and when rings do that the sealing is ruined as the pressure behind them is now lower that the pressure above them, causing them to collapse under the combustion pressure and sometimes resonate and break.

We cannot prevent the ring’s inertia from doing its things, there is a speed where the rings will float, and before that they will progressively start leaking as they get light at TDC and the bottom seal weakens, but the issue can still be mitigated by a sufficiently large end-gap that promotes quick ring back-pressure build up, and by making sure the second compression ring always has a larger gap than the first ring, so any pressure getting past the first ring for whatever reason (gap, flutter, blow-by) does not accumulate in between the two rings, lifting the first one up and making the problem far worse by causing it to collapse. Some piston designs incorporate an intermediate groove between the rings that serves as pressure relief to also help mitigate that issue.

Finally, the manufacturers of even the finest motorsport pistons leave it to the builder to determine the gap. They only offer guidelines, often described as the minimum gap for the application.

The value is often expressed as a factor of the bore diameter. For my new Mahle Motorsport pistons, the recommendation for boosted race application (that is, turbo under maximum sustained load as one can encounter on track racing with long straights at WOT) was 0.007 times the bore for both compression rings, which boils down to 87.5mm x 0.007 = 0.62mm or 0.024” minimum.

The theory is the first ring gets much more heat than the second in boosted applications, so the gap will naturally be larger on the second ring without the need to dial up extra space cold.

Remember than too much gap (within reason) is of little consequence - no increased oil consumption in particular, contrary to the common wisdom, mainly a small loss of efficiency at low engine speed and load - while too little a gap can have important consequences on high-RPM performance and possibly lead to the catastrophic loss of the engine, if the ends ever bind.

Interestingly, even the gapless rings have gaps, they don’t change the sealing fundamentals, just add a twist to it.

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I’ve gapped all my compression rings to 0.007 x bore, maybe a tiny hair less on the top ring and a tiny hair more on the second one - this is all done by hand.

After about 1,000 km (600 miles) so far since the first oil change, my oil level did not move the slightest and the oil is still completely transparent, I have to look for a reflection to see the level on the stick.

I can see the oil changing color a little but at this rate it will take another 3-4000km before it gets to dark-amber, and there is almost nothing in my PCV-side catch can, maybe a few cc (one table spoon) or so, so the rings are apparently sealing well so far.

I hope this post is useful to someone somewhere.

Further readings:

The High-Speed Internal Combustion Engine (Harry R. Ricardo - Ricardo eStore)

Piston ring joint clearance and oil consumption (Motorservice Technipedia)

The Mechanisms of Piston Ring Operation (Tribology - Neale Consulting Engineers)

2020 Motorsport Ring Gap & Filing Instructions (Mahle)

Everything you need to know about ring gap! (Wiseco)

Ring Installation Guide - 2618 pistons (JE)
 

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You are a thorough guy!

I like gapless in NA but have never built a motor for boost. Doing some reading I’m still not sure what I’ll do.
 

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Every six months there should be a multiple choice (guess) quiz for forum members.
In return, Ford should issue certificates of merit, recognizing knowledge and love of the RS.
Discounts on OEM parts and service through a a points system would be nice too.
Wait..what..I was tripping, never mind.
 

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I’ve gapped all my compression rings to 0.007 x bore, maybe a tiny hair less on the top ring and a tiny hair more on the second one - this is all done by hand
Nice informative post 👍👍

So you are at 0.024 inch gaps, may I ask what turbo you are running and how much boost?
 

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Discussion Starter #6
Nice informative post 👍👍

So you are at 0.024 inch gaps, may I ask what turbo you are running and how much boost?
Stock turbo at max reasonable boost, which is 1.85-1.9bar (27.5psi) for as long as I possibly can sustain it. It tapers off a little beyond 6000 or so, still working on that. You cannot get much beyond 1.9 as the MAP sensor clips at 2.0 and it would be crazy to get any closer.
 

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Awesome write-up and so I learned much more than I ever knew about piston rings! It reminded me of when I helped my son rebuild his MazdaSpeed3 engine (first engine rebuild of my life), and I spent an hour staring at this diagram in the shop manual before it finally clicked (and also why the machine shop numbered the piston ring sets!)
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And this is why GM High Performance was able to Boost the ever living crap out of a junk yard stock short-block 4.8L Gen3 engine.

They sized the ring gaps correctly and thus the limiting factor in their test wasn't the strength of the crank, rods, or pistons, but the ability of the OEM ignition system to ignite the mixture.

They took an N/A engine that made about 280hp, and pushed it it made almost 1200hp. If they hadn't set the ring gap correctly it probably would have cracked the walls.
 

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Discussion Starter #11
Awesome write-up and so I learned much more than I ever knew about piston rings! It reminded me of when I helped my son rebuild his MazdaSpeed3 engine (first engine rebuild of my life), and I spent an hour staring at this diagram in the shop manual before it finally clicked (and also why the machine shop numbered the piston ring sets!)
View attachment 349506
Yes I did not mention it but every builder has theis habits about the initial positioning of the gaps. I think the most important is they are not lined up and distributed somewhat evenly. In the shema above they want you to put the gaps as far from each other as possible in the opposite direction. Some say every 60 degree, etc. This is just the initial position, as the rings rotate when the engine runs. Not sure anyone known how fast and in which direction, but the honing pattern has to do something with that.

It is important the rings can rotate, and gunning a cold engine may compromise that: you may get a mini-seizure if the piston expands more rapidly than the cold water-cooled bore and end-up just barely touching. The the result is the rings might not move freely after that if the piston suffers damages (like scuff marks in the ring area). They may wear prematurely and oil consumption may rise with black smoke and less compression.
 

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@axelr I was wondering about that and how much the rings rotated once you put everything back to together and the engine was running. It seems like it is important for compression and oil consumption that the gaps don't overlap, but if the rings rotate how do you prevent this from randomly happening? Or if they do overlap it's just briefly and unlikely all three would overlap at the same time and/or stay that way for very long?

As a funny aside, the wording in the Mazdaspeed engine shop manual confused me even more at the time:
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Soooo...taken at face value it seems to say DON'T align them as shown in the figure? A well placed comma after "aligned" would have helped clear up that confusion! 😂
 

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Discussion Starter #13
Tbh I don’t know at what speed they rotate and in which direction, and it they ever change direction or not.

I’m not sure anyone knows for sure, it’s a little hard to observe. It is just known they rotate, just like the valves, valve tappets, gudgeon pins etc. When those parts seize they show signs of uneven wear and something bad happens sooner or later.

The rings surely align from time to time, just like stars do 😊
 
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