I've been doing a lot of thinking and number crunching lately. With all the engines I have been assembling, a few things have really started to sink in.

One:
We always hear about how crappy the subaru pistons are. People claim WEAK SAUCE! We hear about how some engines will lose compression on a high hp motor with no det. People say "IMPOSSIBLE, YOU HAD DET OR THE PISTON WOULDN'T HAVE FAILED".

I think that's keyboard cowboy talk.

Lets get serious.
Many high hp builds do in fact utilize the factory rings. So basic ignorant thought would be that the rings are strong enough and it's the pistons fault. Well the educated engine builder knows that ring gap is set based on a few parameters. The biggest being bore. The second being the power level that the engine is going to see as well as the mode of power.
Why is that important? Well because like all things in our engines, rings expand and contract based on temperature. The higher the power and heat load, the more they expand.
So lets look at factory ring gaps.

TOP RING: 0.0079- 0.0098"
2nd RING: 0.015- 0.019"
Performance gaps are usually provided to us in a formula. Usually the bore in inches x .005-0.0065 based on power levels. So if we assume 99.5mm and 0.005" then we get 0.0195".
Even the smallest "performance" setting is double the high end of the factory spec.

Now, some people use the factory block for all it's worth and make the same power as the people with the "performance" settings.

This starting to make sense?

Basically, what I think is happening:
People run the factory block at high hp levels. Rings expand, have nowhere to go and break the ring land in the piston. That is after all one of the symptoms of a small ring gap.

This would explain why people will blow ring lands on a det free engine. Just think if the person had the block from the factory with a ring gap of 0.0079". That is freaking TIGHT. 450 hp later, cracked ring lands.

This would also explain why on a lot of motors with broken ring lands, it is really hard to remove the ring from the groove because the ring has mushroomed in certain areas.

I think we need to stop blaming the pistons and start setting the right ring gaps. Anyone with a factory block sitting around about to go into their car should set the ring gaps to at least 0.014" (top ring) and 0.022" (2nd Ring) and report back in 15,000 miles with compression test results.

any takers?

-Dominic

The factory rings are set so tight because the have a lower rate of expansion than your normal aftermarket set of rings. Can't remember exactly the material differences right now, but when I get to the office tomorrow I can post it up.

I don't have any proof, but that doesn't sound right. If that was indeed the case, then why wouldn't JE tell you to run factory specs? They use factory rings.

-Dominic

JE doesn't run factory rings.. I have installed probably a dozen sets of JE's all come with their own rings.

That's funny. My experience is quite the opposite. All the JEs I've installed had a Subaru supplied box and rings....

regardless, i do believe there is merit in this theory. Are you saying you disagree with it?

is a piston ring *tougher* than a ringland when the end gaps meet? that's the question.

I have no basis or knowledge on that, but I wouldn't think so.

How thick are the ringlands compared to the rings?

I would think a ring would *break* if the ends meet under high power.

just talking out loud and discussing. Please don't take it personally.

Ryan :)

But aren't most of the ringlands all breaking in the same location, a spot which correlates with a hot spot in the cylinder head?

oh so thats what your telling yourself for blowing that motor up on the dyno :p

haha just kidding, sounds like you could be onto something though

so dom, are you saying that the thermal expansion of the ring, thickness-wise, is too great for the height of the grooves?
if the gaps are too small you will see butting/mushrooming at the gap and normally find some nice scaring on the cylinder wall...

I could easily see a ring breaking the land if it grew too big for the groove in the piston.

I'm open to listening to any theory but when the rings expand and the ring end gap is too tight you get a deep cylinder scratch where the ring ends touch. We typically do not see this with blown OEM pistons.

Broken ring lands on the OEM pistons are due to detonation and really nothing else. At 400+HP they just can't handle any detonation.

We have run the stock engine to 480-500whp for an entire racing season without issue. The stock pistons are great as long as you don't ping it ;)

Thanks,
Phil
www.elementtuning.com

The is a carry over for another thread I started, but what do you guys think of ceramic coating the crowns?

* Reduces the chance of detonation (via being able to back off the timing a degree or two, due to the improved combustion efficiency of the ceramic coated crown)
* Adds physical strength to the base material of the piston
* Reduce the temperature of the ring land to within the operating specs of the material - due to the heat reflection capabilities of the ceramic coating.

I spoke with 3 coating specialists this week Embee, Swain & Polydyn that all said this three points were correct. (Well one said the second point didn't apply to their process).

Since I will be running my stock STI pistons well within their operating range (say 300whp) I was planning on ceramic coating my stockers as a little cheap insurance. ($20 bucks each to coat, vs. $125 each for forged).

the problem is its not just high hp STi s. my buddys 06 STi had ringland issure with only 15 thouand miles on it running a cobb stg2 map.

as far as the STi goes, I keep reading in different posts that they've been coming with hypereutectic pistons, if that is the case, that alone is a big problem as far as pistons breaking. I've been working with modified chrysler turbo engines since the mid 90's and have seen hypereutectic pistons fail even at stock hp/boost levels, and on those cars thats only 175hp/9psi boost. Those pistons are made so hard that they are brittle and very prone to cracking, typically with detonation/preignition, but thats not always necessary.

But aren't most of the ringlands all breaking in the same location, a spot which correlates with a hot spot in the cylinder head?

I forgot about that. I thought this was the case....nearly the same spot on the pistons.

any other input?

I forgot about that. I thought this was the case....nearly the same spot on the pistons.

any other input?

keep in mind, if that was the hottest cylinder it would also contain the hottest piston/ring.

is a piston ring *tougher* than a ringland when the end gaps meet? that's the question.

I have no basis or knowledge on that, but I wouldn't think so.

How thick are the ringlands compared to the rings?

I would think a ring would *break* if the ends meet under high power.

just talking out loud and discussing. Please don't take it personally.

Ryan :)
The rings are made of a stronger material than the pistons. Also, what if the rings meet and instead of just breaking, they start to BOW. This would effectively thicken them as they are now shaped in an S shape in a square groove. These points of contact would concentrate the force on the piston ring land at that point.

But aren't most of the ringlands all breaking in the same location, a spot which correlates with a hot spot in the cylinder head?

Forgot about that part...

oh so thats what your telling yourself for blowing that motor up on the dyno :p

haha just kidding, sounds like you could be onto something though

It was a rod bearing. Took the motor apart and there was something in the oil. All the cam and rod bearing journals showed some serious grooves. probably why the car only made 238 before dying.

so dom, are you saying that the thermal expansion of the ring, thickness-wise, is too great for the height of the grooves?
if the gaps are too small you will see butting/mushrooming at the gap and normally find some nice scaring on the cylinder wall...

I could easily see a ring breaking the land if it grew too big for the groove in the piston.

I should have specified, the piston rings were sticking IN the grooves. The back side of some of them and the facing sides of others were mushroomed slightly. Not the tip/ends.

I'm open to listening to any theory but when the rings expand and the ring end gap is too tight you get a deep cylinder scratch where the ring ends touch. We typically do not see this with blown OEM pistons.

Broken ring lands on the OEM pistons are due to detonation and really nothing else. At 400+HP they just can't handle any detonation.

We have run the stock engine to 480-500whp for an entire racing season without issue. The stock pistons are great as long as you don't ping it ;)

Thanks,
Phil
www.elementtuning.com (http://www.elementtuning.com)

well we all know that you are some subaru tuning freak, Phil. :p
Seriously though, how hot do you think you are getting your pistons? You run water/meth injection in all the races (don't ya), this could seriously keep the heat down and lower the heat load on the rings and pistons.


the problem is its not just high hp STi s. my buddys 06 STi had ringland issure with only 15 thouand miles on it running a cobb stg2 map.
That right there would make me think it could be something else. Those cobb maps are usually so crazy conservative that I can't see the motor failing from Detonation.







anyway, it was just a theory I thought I'd throw out there. I welcome all the input and ideas. Thanks guys!

-Dominic

The is a carry over for another thread I started, but what do you guys think of ceramic coating the crowns?

* Reduces the chance of detonation (via being able to back off the timing a degree or two, due to the improved combustion efficiency of the ceramic coated crown)
* Adds physical strength to the base material of the piston
* Reduce the temperature of the ring land to within the operating specs of the material - due to the heat reflection capabilities of the ceramic coating.

I spoke with 3 coating specialists this week Embee, Swain & Polydyn that all said this three points were correct. (Well one said the second point didn't apply to their process).

Since I will be running my stock STI pistons well within their operating range (say 300whp) I was planning on ceramic coating my stockers as a little cheap insurance. ($20 bucks each to coat, vs. $125 each for forged).

Yes, it seems that most every builder believes in ceramic coating the piston crown. Many also have a low friction coating added to the sides. I also had my combustion chambers, valve tops and exhaust ports ceramic coated. Ron sent the heads to Swain for this.

The idea was to keep exhaust gases hot and energetic, spooling the turbo a bit quicker. The reduced heat to the heads is a nice side benefit.

Its most definately the pistons fault.

Aftermarket pistons, well, many of them, use the OEM rings, yet they arent having the same problems.

So be it that the OEM pistons are weak, or that they need more space for the rings, they are what is failing. Piston A fails all the time, Piston B doesnt (this is not refering to subarus A and B listings). Piston A obviously has a problem.


And like Phil said, there are other symptoms for ring expansion problems.

But aren't most of the ringlands all breaking in the same location, a spot which correlates with a hot spot in the cylinder head?

yeah, the ring will also have more thermal expansion there...as will the piston...making even less room ;)
so dom's argument would make sense.

Aftermarket pistons, well, many of them, use the OEM rings, yet they arent having the same problems.

well, aftermarket to factory is really apples to oranges without knowing ring the spacing left for thermal expansion (height of the ring groove) and knowing the expansion rates of each material...
I do agree with your statement that regardless of the ring growing too much, its too much for the piston and that does make the piston the failure point. not the ring.

Aren't piston rings and their grooves designed so that there's plenty of oom for combustion gases to get behind the rings and push the ring out from behind in order to better seal against the cylinder wall? This being the case and for a properly built motor, I doubt that one would see the ring expanding in thickness enough to take up all this room and more, breaking the land.

Its most definately the pistons fault.

Aftermarket pistons, well, many of them, use the OEM rings, yet they arent having the same problems.



It is possible to be a combination of both, tight OEM ring end gaps touching at high HP levels along with weak cast OEM pistons. Add in some Det., its to much for the OEM pistons. Ring end gaps should be considered/adjusted for the HP level of the engine.

This may sound like a dumb question, but where do the oe pistons fail, ie at what location around the piston???

well, aftermarket to factory is really apples to oranges without knowing ring the spacing left for thermal expansion (height of the ring groove) and knowing the expansion rates of each material...
.

Very true


I do agree with your statement that regardless of the ring growing too much, its too much for the piston and that does make the piston the failure point. not the ring.

That was my main point anyway.

I'm open to listening to any theory but when the rings expand and the ring end gap is too tight you get a deep cylinder scratch where the ring ends touch. We typically do not see this with blown OEM pistons.

Broken ring lands on the OEM pistons are due to detonation and really nothing else. At 400+HP they just can't handle any detonation.

We have run the stock engine to 480-500whp for an entire racing season without issue. The stock pistons are great as long as you don't ping it ;)

Thanks,
Phil
www.elementtuning.com

I was going to bring this up....but you beat me to it.

Anyhow, the POINT is this:

Det kills the OEM piston. Period. Dot. End of story.

I know of several factory (meaning as it was delivered in stock form) engines in the 400whp to 600whp range being deathly reliable on the street, strip and road course. Thats on either pump gas, pump-meth, pump-water, pump-water+meth, 100oct, C16 and VP Import.

It is all about the TUNING.

I've seen bad tunes kill a piston SUPER FAST on a BONE STOCK CAR. I'm talking under 1,000 miles.

This may sound like a dumb question, but where do the oe pistons fail, ie at what location around the piston???

Imma take a stab at this, and I'll tell you to take it with a grain of salt, and someone will correct me if I'm wrong...

I thought it was around the 6' o'clock position. from 5-7 really. Bottom of the bore/cylinder as the engine sits in the car.

The most common locations I've seen cracked ring lands has always been at the top or bottom of the the bore. Very few have I seen elsewhere, but I have seen them.

I've also seen a few CROWNS break off. 4 engines to be exact.

1. 05 STi, 20G, TBE, EcuTek tune (thats right, nothing else). 5th and 6th gear was creeping past the tuned boost level. Owner continued to drive it around Road Atlanta like that. You can imagine what happened next. Roughly 40% of the crown was gone.

2. 03 WRX. All kinds of junk done to it. Went from EJ205 to 'hybrid' aka EJ257 block w/EJ205 heads. Did not have it tuned, drove around for about 1000 miles on a UTEC that was tuned to the setup prior to the EJ257 block going in (was told it was 'ok' to do this duringthe break in period). Came in with a missfire. No compression (dont remember what cyl). Looked in the chamber with my scope...wow, about 40% of the crown was gone.

3. 06 STi. Turbo XS TBE (the big catless one), Typhoon intake, MBC set to who knows what. Missfire #2. No compression. About 25% of the crown was gone. Customer kept driving it since SOA denied warranty coverage. Later came back locked up w/no oil in the engine. Guy unloaded the car. I wish I would have purchased it as I had a longblock for it at the time....*kicks self in butt*

4. 2005 Baja, 4EAT. No mods when it rolled in. Had signs of a previously installed downpipe (cut and missing heat shields and more), plumbing for a MBC that was poorly restored and did not make boost (restrictor pill in the wrong place), previously installed ebay CAI as all the stock airbox bolts were missing and the fender well was hacked to pieces and some other small bits....missfire #4. No compression. About 30% of the crown was gone.

Some time ago Turbo Diesel trucks had a recall that ask to replace ring and gap them. And the reason was broken ring lands. I never could understand why COBB would install they pistons with stock rings and gaps. And at the same time whoever runs 500whp car using race gas and water injection would have hard time to break they pistons ring lands since low temp in a combustion camber. So tell you a truth I don't care why ring lands keep breaking.

ah well, it was just an idea.

I agree that the pistons are weaker than aftermarked pistons. I never claimed they were as strong. I was just saying that maybe the rings had a larger part in the whole thing than everyone was believing. Maybe if the rings were gapped larger, we would have less broken ring lands. Sorry if it didn't sound that way.

As for the logic "rings aren't breaking, pistons are, so it's the pistons at fault.." that is some interesting thought process. If I hit a piston with a hammer, the problem isn't that the pistons are weaker or the hammer is harder. The problem is that I'm hitting them with a hammer. All I was saying that is if the rings were to bend and put excessive amounts of pressure on a small point because they were gapped too small, it could cause failure of the pistons.

A few things we can all agree on:
1. OEM pistons are weak.
2. Detonation is bad.

Things that my exaggerate the issue:
1. small ring gaps for high hp <- untested, unproven theory and the topic of this thread.


Thanks for participating. I like to keep the creative juices flowing with new ideas.

-Dominic

One thing I question, and maybe it's OT, but are forged pistons such as CP/JE/Weisco/etc actually softer than OEM metal? I understand OEM is very hard and thus, brittle. just like peanut brittle :p

Is softer the correct word for the metal? Do the ringlands deform more? Hearsay says the aftermaket forged pistons can take significantly more det. than OEM pistons.

anyone know what happens to a piston when det occurs in a combustion chamber. a camera would be cool, like those valve spring video's that are out there. I never knew how much they actually rotated. :wow:

modaddict....

Your are thinking of what is called Elastic Modulus (How much it will give, and has a very small range) , which is directly related to Tensile, Yield , and Shear Strength (how long and how much force it will put up with, and can have a huge range depending on the alloy) ...

You are correct in the fact the forged pistons have more give, but the are harder at the same time...

I am no expert on it, but it does make some sense to me....

Your are thinking of what is called Elastic Modulus...
That's my understanding as well. Both types of forged pistons have substantially greater ability to temporarily deform, and not break in the process. However, given severe enough detonation, they will break too. Just ask ejh25, who had a Cosy 2618 alloy piston break into bb's and aluminum dust on the dyno...I was impressed.

That's my understanding as well. Both types of forged pistons have substantially greater ability to temporarily deform, and not break in the process. However, given severe enough detonation, they will break too. Just ask ejh25, who had a Cosy 2618 alloy piston break into bb's and aluminum dust on the dyno...I was impressed.

Video?

So forged pistons are harder with a better ability to temporarily deform than OEM pistons? Interesting.

I think there may be some validity to this theory. If you don't blame the rings you blame the ring gap it makes sense. Piston tops have been separated from the wristpin due to ring gaps being too small and the rings expanding and butting up against itself and the bore. You say the ring faces where sort of mushroomed, I think that would require more force than just spring tension and combustion gasses pushing outward on the rings.

modaddict....

Your are thinking of what is called Elastic Modulus (How much it will give, and has a very small range) , which is directly related to Tensile, Yield , and Shear Strength (how long and how much force it will put up with, and can have a huge range depending on the alloy) ...

You are correct in the fact the forged pistons have more give, but the are harder at the same time...

I am no expert on it, but it does make some sense to me....

Toughness is the term. Fracture toughness, Kc, is a measure of a metals ability to take a beating without breaking. More brittle materials have a lower fracture toughness but tend have a higher tensile string which is a measure of the materials strength.

A few things we can all agree on:
1. OEM pistons are weak.
2. Detonation is bad.


Det bad? Certainly.

But the OEM pistons weak? Thats not the correct wording. I've seen many EJ257/EJ255 making 500~600whp and they have lasted quite a while and never had a broken piston.

Brittle? F*** yes. They have no give to them due to their manufacturing process. But does that make them weak? Well, its in the eyes of the beholder. I think they are perfectly fine to 600whp (since thats the 'limit' it seems for stock heads/cams so far) so long as you have a tuner with some common sense. If you dont, well, you'd best get some 4032/2618 pistons FAST. Oh, and rods too....det breaks rods as well.

I still say oem pistons are weak in that they don't have toughness...which is an engineering term.
Their modulus of elasticity is pretty much the yeild point too (guess based off of material type used)...really would be interested in seeing a stress strain diagram on the ring lands in comparison to 4032 and 2618...
someone wanna donate some pistons? :D

I could donate some stock pistons...?

Imma take a stab at this, and I'll tell you to take it with a grain of salt, and someone will correct me if I'm wrong...

I thought it was around the 6' o'clock position. from 5-7 really. Bottom of the bore/cylinder as the engine sits in the car.

This is where mine broke last time...(#4)
Which is guess is the stereotypical "6 o'clock" position.

http://i27.photobucket.com/albums/c171/Dudley302/4broken.jpg


But another piston (#2) had severe damage (cracks). Which somehow magically lined DIRECTLY up with the ring gap of the upper ring. And the other cracks were on the opposide side. And neither piston/cylinder/head showed any signs of overheating, detonation, or being lean...

So I think this ring cracking the piston theory holds some water..
http://i27.photobucket.com/albums/c171/Dudley302/2cracks.jpg
http://i27.photobucket.com/albums/c171/Dudley302/2morecracks.jpg

neither piston/cylinder/head showed any signs of overheating, detonation, or being lean...

So I think this ring cracking the piston theory holds some water.[/IMG]

Dunno about that...

I've have taken apart engines that were det'd to the moon and back and they looked 'normal.'

Just keep in mind that you have to have multiple facts to back up a theory and while it may hold truth in some applications, in the Sti application we are just not seeing the cylinder scratching. If the vertical cylinder scratches were common along with the broken pistons then I would say it's possible.

Often if the customer does not continue to drive their car with the damaged pistons the bores rarely need more than a hone. When the rings stick it can leave a gouge so deep it cannot be honed and in some cases not even bored.

Ring end gap sizing takes a little trial and error and we've done our fair share of it with our race engines. On one side you want the looser gap to prevent ring sticking but on the other hand a loose ring end gap will cause excessive blow-by until the motor reaches the extreme cylinder temps a looser gap was designed for. The piston choice also makes a difference. A nice tight top ring works great on the OEM piston even at 500+ hp but it's a little tight for a 2618 forged piston. It's a doubled edged sword for sure. We tend to make the decision based on our personal testing, the customer's build, and the usage more than using a rule of thumb.

Thanks,
Phil
www.elementtuning.com

It's a doubled edged sword for sure. We tend to make the decision based on our personal testing, the customer's build, and the usage more than using a rule of thumb.

Yup, its a sliding scale for sure.

I was going to bring this up....but you beat me to it.

Anyhow, the POINT is this:

Det kills the OEM piston. Period. Dot. End of story.

I know of several factory (meaning as it was delivered in stock form) engines in the 400whp to 600whp range being deathly reliable on the street, strip and road course. Thats on either pump gas, pump-meth, pump-water, pump-water+meth, 100oct, C16 and VP Import.

It is all about the TUNING.

I've seen bad tunes kill a piston SUPER FAST on a BONE STOCK CAR. I'm talking under 1,000 miles.


Assuming that the car is tuned by reflash, wouldn't the car ecu pull timing upon det? Worse case scenerio, the car will go into high det fuel map?

Been hearing and know cases of broken ringlands with the car ecu still sitting pretty at 1 IAM.

Stumped!

Assuming that the car is tuned by reflash, wouldn't the car ecu pull timing upon det? Worse case scenerio, the car will go into high det fuel map?

Been hearing and know cases of broken ringlands with the car ecu still sitting pretty at 1 IAM.

Stumped!

In order for the ECU to pull timing due to a det event it has to SEE the det event happen. When the det happens, well, its too late.

Nice pictures...
I am glad that you posted such detailed pics...

I have seen this in boosted applications, and I think I know what it is caused by... I have seen it with forged pistons as well...

http://i27.photobucket.com/albums/c171/Dudley302/2morecracks.jpg

Did you notice that a lot of the carbon is missing from in between compression ring #1 and #2 ???

It is literally blown off of the piston...

I have seen this in superchargered engines, and only two things can cause this...

The first is a high rpm misfire... Which leads to what actually breaks it...

After the misfire, the compression rings lift off of the cylinder wall from the vacuum that is pulled in the cylinder on what should be a compression stroke...

When this happens, fuel and oil get in between the compression rings...

Then, the next compression stroke event either, sets off the now prone to ignite mixture between the rings, or transfers the ignition shock to the land...
Ie Hydralic shock, which will have the most force at the gap...
If the piston and rings can not relieve, or withstand either senario, something has to give...
I have seen this "give" at the ring gap more than once in the past...
The only "fixes" I have ever seen for this was a different shape ring groove and ring...
Top ring porting to the face of the piston works as well, but that is not for street use....

The second is piston rock, pinching or binding the rings into the grooves on one side of the piston, unevenly side loading them, which leads right back to a similar failure from lifting the rings off the cylinder...

I am sure that there are other things that can cause this as well...
But these two are the most common causes that I have seen...

shvrdavid,

I like to run on the lean side. Is there a possibility of tuning the car on the richer side, think in the 10's AFR (with a boatload of timing).....and having the fuel "back up" (HC's) in the cylinder and get into the rings/ringlands.....which will eventually light off on a power stroke? (and thus breaking a ringland)

...The second is piston rock...
Hmmm...with the smearing on the skirt, that makes some sense. That piston, at some time, sure looks like it made some pretty serious contact with the cylinder wall.

Nice pictures...
I am glad that you posted such detailed pics...

I have seen this in boosted applications, and I think I know what it is caused by... I have seen it with forged pistons as well...

http://i27.photobucket.com/albums/c171/Dudley302/2morecracks.jpg

.

I was noticing the mushroomed ring in this picture. The last half inch right before the gap is very deformed.

-Dominic

That piston is very worn on the skirt. It actually had audible "piston slap" for the first ~20 minutes that the car ran, then it would subside. I knew it was probably a broken/worn skirt so I actually let the car warm up 20 minutes every time I drove it for a while now. So I believe that its definately possible that piston "rock" caused the failure on this particular piston. I actually pulled the motor running, but it was starting to lose compression, as you can see why.

This is how bad it is:
http://i27.photobucket.com/albums/c171/Dudley302/2skirt.jpg

If you have the build notes handy, what cold P-SW clearance was this jug set to? Also, was there any damage to the rod bearing or the pin?

What are the cylinder walls looking like when the ring lands break? If a ring is too tight an d stacks up I'd expect the cylinder bore to be pretty scraped up all the way around. If you're not seeing that I'd have a hard time blaming the piston failure on ring end gap. Ring land breakage is seen across the piston engine landscape regardless of make as a failure from detonation.

All the ones I've seen the cylinder is totally fine. I even had one that was run long enough to burn all the way through the upper compression ring and the piston was almost in a D shape and it still didnt do anything to the cylinder wall.

If you have the build notes handy, what cold P-SW clearance was this jug set to? Also, was there any damage to the rod bearing or the pin?

That was a stock 05 sti shortblock with ~26k miles on it. Connecting rod bearings showed a little wear towards the small end, but nothing crazy. Wrist pin spins and goes in and out with clearance issues, con rod pin bushings are also not worn.

All the ones I've seen the cylinder is totally fine. I even had one that was run long enough to burn all the way through the upper compression ring and the piston was almost in a D shape and it still didnt do anything to the cylinder wall.

Which would seem to indicate a piston failure due to det not the ring 'stacking' due to too tight end gap.

That was a stock 05 sti shortblock...
Huh, that looked like a Mahle for a second. With a stock clearance of .0004", I think we can rule out piston rock, and I guess we're back at square one: detonation.

But aren't most of the ringlands all breaking in the same location, a spot which correlates with a hot spot in the cylinder head?

it would make sense that the ringlands break at the hottest /weakest point caused by the expanded rings...

it would make sense that the ringlands break at the hottest /weakest point caused by the expanded rings...

that would assume the rings would be assembled at the same spot, or navigate tot he same spot.

highly unlikely.




Anyone think that piston expanded too much at one time and caused the aluminum transfer. I bet that jug was tight and at high temps......ouch.....

that would assume the rings would be assembled at the same spot, or navigate tot he same spot.

highly unlikely.




Anyone think that piston expanded too much at one time and caused the aluminum transfer. I bet that jug was tight and at high temps......ouch.....

the energy from the expanded ring would not stay in one spot, its going to move around the piston untill something gives. jmo

...Anyone think that piston expanded too much at one time and caused the aluminum transfer. I bet that jug was tight and at high temps......ouch.....
You wouldn't think so on a stocker at that power level, but that is exactly what a stuck (ie, tight) piston looks like. I'm thinking detonation on one side maybe caused the piston to deflect off-plane just a bit for a few passes? That'd be all it'd take. Anyone else?

shvrdavid,

I like to run on the lean side. Is there a possibility of tuning the car on the richer side, think in the 10's AFR (with a boatload of timing).....and having the fuel "back up" (HC's) in the cylinder and get into the rings/ringlands.....which will eventually light off on a power stroke? (and thus breaking a ringland)


Yes, I am sure that it is possible...

Long term will pack the ring grooves with carbon... (think high mileage engine, but real quick...)

Running that that rich can also wash the oil rings...

On the piston expansion part... If it had cold slap, the clearance was way above the factory spec...

Usually if a piston expands enough to start what I refer to as "Black Death" your trust load sides of the cylinder will be scored at the bottom of the bore...

Turbo_Mike, can you post pictures of the bore of the engine that you took the pistons out of...??? I am curious to see if your bores did that...

...Turbo_Mike, can you post pictures of the bore of the engine that you took the pistons out of...??? I am curious to see if your bores did that...
+1. Everytime I've had a piston smeared like that, the jug had to be either bored over, or replaced.

here are some top ring gap recommendations from Weisco for a 4" bore.


high-performance street/strip .018"
street-moderate turbo/nitrous .020"
late model stock .022"
circle track/drag race .024"
blown race only .026"
nitrous race only .028"


**** stock ring end gaps and their never-ending quest for PZEV and ultimate gas mileage at the cost of blown pistons.

Turbo_Mike, can you post pictures of the bore of the engine that you took the pistons out of...??? I am curious to see if your bores did that...

Sorry, I missed that one... I'll grab a pic of it tonight. I havent done anything with that block yet. It actually doesnt look as bad as you would think. I was considering going back with stock bore with this one, but I convinced myself it was a bad idea and its going to be 100mm....

**** stock ring end gaps and their never-ending quest for PZEV and ultimate gas mileage at the cost of blown pistons.

+12345 :(

I am convinced this is why OEM pistons are so unforgiving. Everyone can argue that it's "all in the tune", but at the end of the day, you are still going to be limited by weak piston alloy and gap measurements that aren't intended for big horsepower/tuning applications. Not saying you can't make good power on stock cast alloy... it's just difficult to do so with longevity in mind.

why would anyone think a factory piston in a engine rated at 300 chp would
handle almost twice the output as intended to last as long as a stock?

yeah they can handle more more power. but don't fool yourself and believe
that the pistons will last as long as stock.

stock pistons don't even last in stock set ups from the factory.
even after proper dyno tuning they will eventually crack/break.
the question when will they go bye-bye.

why would anyone think a factory piston in a engine rated at 300 chp would
handle almost twice the output as intended to last as long as a stock?


Because!!! If supras can do it so can every other stock internal'd import ;)
We all know everyone wants to be just like a supra, just with more traction