Yellow 6.5 Tear down details VERY LONG READ (but pictures)...

[john8662]

First off, images that appear in the posts are usually linked to a larger version of the image to see some more detail, some images were small enough to fit. Second, I had to break this post up because of the Max no. of images per post was exceeded.


Well, after much debate with myself, I got the nerve to go ahead and pull the 6.5 back out of the burb. Removing an engine the 2nd time from this vehicle went much quicker this time.

Anyways, as we left off last, the engine suffered a failure that was self-inflicted (but not totally). I had done a quick-fix that ended up damaging the engine. Basically, I had two stripped out center lower intake mounting holes, and instead of removing the intake, then drilling the stripped out holes bigger then tapping those holes to receive a helicoil, I did something else. I elected to take 1/4-20 all-thread and cut it to two 3" lengths. Then I took the studs and inserted them into the holes and installed one nut, followed by a regular 1/4-20 nut to "lock" the first nut from coming loose and then going into the intake on each stud. Well, as I now know, when tightening the upper nuts that will hold the upper intake manifold to the lower, the action would in turn loosen the nuts below (without me knowing or seeing). Anyways, on the second test drive after a turbo swap the two loose nuts became free in the intake and found their favorite place to go. The engine didn't fail on the start before the second test drive. Basically I took a drive around the block near the shop I work on my trucks, I decided to give it a little throttle (1/2) to get some boost after the transmission had shifted into 2nd gear. As soon as I saw 6psi on the gauge, I heard a nasty noise. I immediately let off the throttle because I knew something had broken, but just assumed the engine would just die. But it kept running, but had a huffing noise similar to what a gas engine would sound like that had lost a plug. I limped it back to the shop. Basically, what happened, is the nuts were probably riding on the top of the valve train until they had sufficient pressure to be forced underneath a valve and into a cylinder, the boost..

At least I got to drive it twice...

Anyways, now to the tear down. So upon teardown I wanted to take a look at where the (2) 1/4-20 nuts went and what damage they caused. Well, I had a good hunch that at least one of the nuts ended up somewhere on the cylinder bank that houses #'s 2-8 (turbo side). So this is the first head that I removed. Well, I was right, I found both of the nuts (or what was left of them) in cylinder #6. The two nuts had been compressed into one piece. At one point in their journey, one of them had managed to hang the intake valve, to the point that the piston slapped the valve.

Here is what I found when removing the head.

http://members.cox.net/acmdsl/www/sub/cat/no6_found2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no6_found.jpg)

Here is what I found cleaned up and beside the "original" nut for reference.


http://members.cox.net/acmdsl/www/sub/cat/nuts.jpg


So, at this point, I removed the prize, and decided to rotate the engine to see what kind of damage occurred to the cylinder wall, because from the top side, the piston appeared to be intact. Well, I rotated the engine, and all the pistons went up and then back down. That is, except for #6, so I knew more damage lay below. The cylinder head appeared to be untouched though.

http://members.cox.net/acmdsl/www/sub/cat/no6headclean2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no6headclean.jpg)


Now, what about the coolant in the cylinder theory or compression bleeding sound that I heard? Well folks, no coolant in any cylinders, none. Why the compression noise external to the engine? The pre-combustion cup was beat up badly, and I think that broke the seal on the head gasket or stretched the head bolts, so the compression from at least one cylinder was being bled outside of the engine.

At this point I figured that I'd pull the other cylinder head, for one thing, to make sure everything was ok there. Also, so that the engine would be a lot easier to rotate on the engine stand.

Well upon removing the other cylinder head, I found more damage, cylinder #3 was really pecked up. There was a very small amount of material in that cylinder that really pecked up BOTH the head and the piston.

http://members.cox.net/acmdsl/www/sub/cat/no3_debris.jpg

http://members.cox.net/acmdsl/www/sub/cat/no3_peckedinstall2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no3_peckedinstall.jpg)

http://members.cox.net/acmdsl/www/sub/cat/no3headdirty2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no3headdirty.jpg)

[john8662]

Now, I'm confused, I thought that I had found the prize. Now the debate. How could parts of the nuts get back up into the intake and back into the other side of the engine? I don't buy that it did. The next point is how badly the small piece of material damaged the head and valve surfaces, yet the piece of metal isn't deformed. I think that there was something else in this engine from the very beginning. I do think this is the case, because I did have a very slight noise in the engine when it was running the first time. Hmm... A couple of points and then I'll continue. Whatever material was in that cylinder was a very hard piece of metal, probably hardened. The head material is a harder material than that of soft steel that the nuts were made of. The valves were damaged and pitted as well, which are hardened steel. So what ever part of material was in that cylinder was a hardened piece of metal, and it wasn't altered in shape (that i can tell) through it's life in the 6.5. Also, when you look back at the nut material left over in Cylinder 6, the metal was really softer and was shapeable by pressure and heat in the cylinder. Also, the piece of material in the cylinder IS small enough as to not cause the engine to bind due to the lower 18:1 C/R and the piston not coming up as high in the bore. Finally, the amount of damage present on the piston to me indicates that the piece has been in the cylinder form the very first start, which was pre-nut incident. But, what do I know?

Here are some cleaned pictures of the head.

http://members.cox.net/acmdsl/www/sub/cat/no3headclose.jpg


http://members.cox.net/acmdsl/www/sub/cat/no3headcleanflash2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no3headcleanflash.jpg)


Here is what was in Cylinder #3

http://members.cox.net/acmdsl/www/sub/cat/no3shrapneldetail.jpg

http://members.cox.net/acmdsl/www/sub/cat/no3headshrapnelmeasure.jpg

http://members.cox.net/acmdsl/www/sub/cat/no3headshrapnel.jpg


Ok, now the bottom end. I pulled my nicely glued pan off the engine block to find that everything looked pretty well normal, that is until I looked closely at #6. I then proceeded to pull the piston out. Basically the piston had split on the third ring groove from a crack that started at the wrist pin boss of the piston. Luckily, the broken part of the piston rode at the top of the cylinder bore while the lower part of the piston including the pin and skirt assembly stayed on the rod. I don't know how, but the wrist pin stayed on what was left of the piston as well as the retainer clips and all. What

[john8662]

Next, cylinder #3 had to be removed to see if the piston was further damaged. Piston #3 had cracked on the wrist pin boss just as #6 did, but the piston had not split yet. Further inspection of the piston revealed a crack that went across the entire underside of the piston, so it was going to split just like #6 did overtime.

http://members.cox.net/acmdsl/www/sub/cat/no3_crackedpinboss2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no3_crackedpinboss.jpg)


Top of that piston...

http://members.cox.net/acmdsl/www/sub/cat/no3_wipedpecked2.jpg (http://members.cox.net/acmdsl/www/sub/cat/no3_wipedpecked.jpg)


So what else did I find?

Well..

Besides Cylinder #3 being damaged that was unexpected after finding my damage there were other things.

Cylinder #4 beside #6 has a deep score mark in the bore, that

[markrinker]

It seems you are taking all of this in stride. I don't believe my abilities mechanically would ever allow me to get through a complete rebuild as you are performing.

I CAN haul things and plow snow, soo I write big checks when I have to!

Good luck!

[stingthieves]

Quote =Now, I'm confused, I thought that I had found the prize. Now the debate. How could parts of the nuts get back up into the intake and back into the other side of the engine?

You must believe it now - There it is
-> I think the tech term is called "REVERSION" - Cylinders will swap gases and other "prizes"! Highly skilled port and polish jobs will limit reversion and cheep pocket jobs can make it worst.

[patrick m.]

any chance that little piece is a roll-pin (may be called spring tennsion pin in some parts).

or part of a roll-pin, certainly would be hard enough to mark cast iron and steel.

[rjwest]

with a hung valve, the intake now becomes an exhaust valve to feed other open intake vales.\\very common failure in airplane engines...can happen with either valve hung ( intake or exhaust )

Hung intake is combustion gas or pumping pressure from that cylinder blowing back into the intake runners,

Hung exhaust valve allows exhaust pressure from other exhausting valves into cylinder, when intake
opens cylinder exhausts back through intake
again into intake runner.

[Alf]

I gather that this engine has been rebuilt recently?
If so was it a re-ring job? Installing new rings on used pistions and honing the bores?

IF SO, consider this
I have seen several cases where some one used a ring groove scraper over zelously, actualy removing metal from the botom of the groves.
This will put stress risers in the piston at the bottom of the groove, add alittle detonation (or a nut between a valve and its seat, causing valve to piston contact) and Tada! you have a split piston!
My years as an air cooled vw mech have shown me many broken exhaust valves. In the short time it takes from the valve head breaking off, to engine stop, the debri would be in All of the combustion cambers due to reversion of air flow in the intake manifold.

[Alf]

Definetly spit balling, thinking out loud,,,It's also possible that with all the havoc going on in the intake tract, the shock waves or even one of the nuts might have hit the back of the turbo blades and broke something off, adding to the debri. Don't know how long it took to get back to the shop after the "big bang" but it wouldnt take very long for all the secondary damage to ocour. Like less than a minute of running time.

Any way, you have my deepest sympathy.

[dieseldummy]

John,
As I've mentioned before, I've had a lot of 6.5 damage similar to yours and it has always been isolated to one cylinder... When I had an intake valve break it ran for a while and beat up the #4 cylinder pretty good. None of the valve particles made it into other cylinders. Also the damge to the #3 on your engine looks different than the damage in #6... The material in #6 was apparently softer since it didn't mark-up the entire cyl head and piston, just kindof smashed into a ball. The stuff in #3 looks like a hard material since it made it mark on everything... I'm interested to know why engines from the depot always seem to have scoring no matter what as well.

Justin

[Dvldog 8793]

Howdy
The over zealouse cleaning of the ring grooves would be the most likely reason for the split pistons. I've seen ash trays made out of similar pistons. I would bett good money that if you used a crack developer you wuold find the same thing waiting to happen to other pistons as well. The small amount of HARD material could be part of a snap ring from the wrist pins. A roll pin would probably vaporize into metal dust due to the thin wall and brittle material. Also maybe it could part of a ring? OR it could be a true FOD from a mechanics tool box. The scoring almost has to be from improper rings/assembly. It REALLY sounds like you are having an adventure! I gotta say you are WAY more patient than I!!! tongue.gif
Keep the info coming!
L8r
Conley

[NH2112]

Had something similar happen at a previous job, where a 1/4"-20 nut somehow found its way into the intake manifold of a Perkins 4.236 going into a Sellick forklift. The tech got everything hooked up, started the engine, and PUFF PUFF PUFF out the intake. The nut was actually embedded in the piston crown, and must have gotten caught going through the valve because the valve was warped. EPS (the rebuilder) sent a new head, piston, and I think a rod too in case the old one had gotten bent, but the tech got hit with a serious redo because "he should have checked for debris in the engine before installing it."

[restoguy]

As said before, reversion happens! I didn't know that's what it was called, but it happened to a 327 that in a '67 camaro that I helped restore. Someone dropped a nut in the intake by accident and we found out after the fact. It beat up pistons on both sides. At the time I didn't believe it, I just assumed there was more than one nut and one managed to pass out the exhaust. But a couple years later I got involved with a circle track racer and he told me horror stories about engines comming apart at full thorttle doing 7000rpm. He said that once they even found a piston ring(most of one anyway) in the plenum of the intake!! How does that even fit up the runner?! That peticular engine, he said, was a total loss. Only the carb was savable, it destroyed everything else. Including the intake, starter, and water pump! I would have like to watch it happen. From behind ballistic glass and on someone elses dime, of course!
By the way, that piece from the second cylinder, the hard piece, looks familiar. I swear I've seen something just like that before. If I think of where I saw it I'll post it for sure. I admire your tenacity, keep fordging ahead!

[john8662]

Hmm, food for thought.

The reversion as it's put could possibly be more of an issue an a gas engine where you actually have vacuum present from other cylinders to pull debris out of a damaged cylinder that has a valve open. But, in our Diesel here, there is little vacuum (as compared to the gas engines presented so far) that material from swapping from cylinders is a little less likely.

I think that a test of the alloy of the metal in both cylinders that were damanged would be in order, that is, if Avant would even consider such a thing. Right now, it's completly on me... Kind of like, nut down the hole, whatever you find it's your fault no matter what it is kind of deal...

Oh, the crank needs work too now, as I pulled piston #4 (the one that didn't have any debris but had scoring) and the bearing and journal on the crank are grooved, possibly debris in the oil passage or an unclean install. The bottom end of this engine was taken apart a few times during the assembly due to an issue encountered with the splayed mains, makes me wonder now.

I didn't assemble this engine, but I sure did do it in :rolleyes:

[ 11-29-2005, 06:38 PM: Message edited by: john8662 ]

[Alf]

It's not the vacume sucking the debri into other clynders, it's the presure from combustion/compresion blowing it out of the source cylinder past a damaged or hung intake valve
Now, in a gas engine the spark is going to ignite the air fuel mixture(regardless of how much of a compresion leak a damaged intake causes) and blow crap back through the hung open valve into the intake where it gets drawn into another cylinder.
In a diesel engine ignition will still take place in a cylinder with a damaged valve (up to a point) because it's up to temp(and there's hot bits of debri rattling around in there) and the engine is turning cause the other clyinders are still firing. The presure generated by ignition of the air fuel mix will easily overcome the boost pressure in the intake manifold. Even without ignition, a pistion pushing up on a cylnder full of air, fuel and debri will still overcome boost pressure in the intake manifold. Remember the Huffing noise? That was coming from the intake.
I know it's kinda hard to follow, but when you've seen it happen many times it makes sense.

[LanduytG]

I have my opinion but will keep it to myself for now. Mic the bore John top middle and bottom and lets see what it is.

Greg

[Kennedy]

My question is what is the compression height(ie recession/protrison) of the pistons on a good cylinder? In other words how far down the bore do they lie at TDC? There sure looks to be a lot if the Ricardo recess left in the dome for a machined 18:1...

[john8662]

Kennedy,

These were supposed to be "real" 18:1 pistons, where the piston was actually cast with a higher wrist pin height. These were not shaved.

I'm going to borrow a dial gauge and magnetic stand to check that very detail this weekend. I'm also hoping my ordered mic shows up before the weekend, for the final diagnosis to check some things so I can figure out what the next move will be.

[Kennedy]

So they are not Mahle pistons then.

[john8662]

They are Mahle Pistons. Everything posted above is what I've been told by Avant.

[Kennedy]

Mahle has not released 18:1 pistons for the 6.5 yet. I have very strong connections with Mahle and we've discussed this numerous times when someone claimed to have Mahle 18:1 pistons for sale. Might be worth furher investigation on your part.

PS a Mahle piston will say Mahle in raised letters up on the inside near the pin bore as well as IND BRASIL.

Mahle makes standard compression height and .010" topped pistons to allow for decking of the block...

[ 11-30-2005, 01:23 PM: Message edited by: kennedy ]

[grape]

the only piston I've heard of that had the pin moved was NOT an 18-1 it was for the deck being cut .010 and the pin was moved up to compensate.

[john8662]

Well, I can't ask Avant the question again, as they're not answering the phone or returning the calls, it's a lost cause there. Maybe you might have better luck?

But I was told numerous times that they pistons were not shaved and were the custom pistons. But, while rotating the engine over, I can tell they're reduced height (either topped or wrist pin height) because they don't come close to protruding from the deck. Though, it wouldn't surprise me if they're not what I was told.

[Kennedy]

Do they say Mahle?

The adjustment can be made somewhat in the connecting rod also...

[john8662]

Yes, I double checked, the say MAHLE on the underside of the piston and are made in Brazil.

[arveetek]

John,

I'm sure sorry to see and hear of your engine damage. It makes me sick to my stomach thinking of all the work and money you've put into this engine, just to have it pretty much trashed at start up. I wish there was something I could say or do to help out, but I can't think of anything.

As you know, Benny Avant made me custom pistons. I wanted to lower the compression a little, but not too much. Benny said he would start with pistons that had the wrist pin moved up by .010", which would drop the compression one point. Then, to lower the compression a bit more, they shaved another .010" off the top of the pistons. The tops were then ceramic-coated. These pistons were stamped "Mahle" as well, but came in a Clevite box.

Benny told me that for every .010" of reduced piston height, one point is dropped off the CR. So, if my engine started life with 21.75:1 CR, then it should now have around 19.75:1.

My modified pistons sit at .015" below the top of the deck at TDC. I believe that stock pistons stick out the top of the deck about .005". This might help you compare your pistons to mine.

BTW...since I didn't have a dial indicator I measured how far down the piston sat in the bore by laying a known straightedge across the top of the deck and then moving the piston to TDC, and then using feeler guages between the piston and straightedge 'til I found the one that fit snug, but didn't lift the straightedge off the deck.

Casey

[ 12-01-2005, 06:23 AM: Message edited by: arveetek ]

[G. Gearloose]

Quite the mess, good luck!
That little hardened nubby looks like a piston ring locator, like i've seen on 2-stroke bike pistons. Keeps the gaps oriented opposite each other.

[eracers999]

John:

I've kicked this whole thing around in my head a while and here is my take on it.

I have never seen a engine pull the tops off the pistons that it couldent be traced back to the ring gaps being too tight. The top of the piston had to be anchored down for it to come off. If this could not be proven then i would suspect that the pistons came from another engine, and the rings were tight in that one.

The mess inside this engine of yours looks far more than just a couple of nut's getting loose in it. Just dont add up. Maybe MP should get involved due to the fact that they wont communicate.

Best to ya!! Kent

[BobND]

Looks to me like the edge of the THICK piston top smacked a non-compressable object (nut), causing the piston to "rock" in the bore around the axis of the wrist pin, shearing the top and bottom of the piston apart... the piston top was too strong to fail, as well as the skirt, and the piston broke at a weak point.

Getting a foreign object in a diesel is different than in a gasser because there is so much less room due to high compression, and the piston top is so much thicker to withstand the stress of diesel operation.

When you think about it, if the rod didn't fail, or the cylinder head, or the top of the piston, something had to "give", (or the engine wouild have stopped "dead") and it did... the piston rocked in the bore, and broke.

[big61fourby]

"Getting a foreign object in a diesel is different than in a gasser because there is so much less room due to high compression, and the piston top is so much thicker to withstand the stress of diesel operation.

When you think about it, if the rod didn't fail, or the cylinder head, or the top of the piston, something had to "give", (or the engine wouild have stopped "dead") and it did... the piston rocked in the bore, and broke. "

Not trying to get way off topic, but does this mean that the head/block casting for the 6.5 are different from a gasser? My roomate had a 350 that sheared a valve head clean off, and within several revolutions destroyed the head, block and piston. Upon teardown, all the other cylinders were fine...

[BobND]

I KNOW the piston tops are certainly thicker than a gasser's, and I strongly suspect the thickness of the head's cast iron over the bore has to be thicker, and it's a safe bet that a little more "meat" is required in the cylinder walls of a diesel to withstand the shock of the diesel's combustion.

(Oh, maybe GM forgot that last one, for many of these blocks as have cracked!)

[DmaxMaverick]

Originally posted by BobND:
....and it's a safe bet that a little more "meat" is required in the cylinder walls of a diesel to withstand the shock of the diesel's combustion.

(Oh, maybe GM forgot that last one, for many of these blocks as have cracked!) I agree with all but that last line. Of all the Diesel engines (light duty/light-medium duty) until the Powerstroke and Cummins, GM's cylinders were the least problematic. 6.9L and 7.3L engines reguire SCA's to prevent coolant cavitation, caused by thin cylinder walls. None of GM's Diesel engines ever required any SCA, due to greater cylinder wall thickness. The cracks (both block and head) are caused by weaknesses in other areas. GM should have addressed this at the first sign, as well as Stanadyne's Holy Grail EFI. On the other hand....if they had fixed all that, we may not have such a beast in the Duramax.

But, to comment on the topic, and in regards to the 350 with the sheared valve...A valve is a little harder (I think...) and larger than a 1/4" grade 2 nut. Something had to give, but it took a little longer than it would have if it had been a valve. Any Diesel engine worth its salt will have heavier castings and rotational components, compared to a gas engine.

[twaddle]

Sorry to hear of your bad luck.
I agree with BobND and I am surprised that some of the experienced haven't said so sooner, the pistons have rocked due being stopped from reaching their full TDC with the nuts in the cylinder, the force on the skirt HAS caused the cracks at the weakest point of the piston leading to failure.
Absolutely nothing to due with rings being tight in the grooves.

The transferring of the nuts back and forth into different cylinders can happen when the valve catches the nut and ejects it out the port back into the intake manifold or into another cylinders port.

Are the Avante Brothers being blamed for any of this engine failure?
It sure seems to be coming across that way from some of the comments?

Regards

Jim Twaddle
Biggar, Scotland

[rjschoolcraft]

Frankly, the rocking idea doesn't make sense to me at the moment. If it had rocked enough to form the cracks and cause the failure that we're seeing here, the skirts would have been lying in the oil pan long before the top pulled off of the piston.

I had a 350 gasser that I did some racing with a long time ago. A sore loser sabotaged it by throwing a nut down the intake. When it finally entered the cylinder, the piston rocked as you describe, but broke a skirt and pushed sideways cracking the bore in three places.

The tops of John's pistons came (are coming) off due to a stress riser in the ring groove. It's hard to see from the posted photos, but it sure looks like there are tensile initiation sites on the right of the lower portion of the piston.

I can't see indication of compressive failure.

[twaddle]

If a nut is at a position 90degrees to the centre line of the gudgeon pin and the crankshaft tries to push the piston to its TDC the piston will rock while trying to reach TDC due to the nut preventing full TDC travel, twisting the piston and pushing the piston skirt at one side tighter against the cylinder wall.
You now have the skirt jamming against the cyl wall and the crankshaft pushing the piston crown away from the skirt leading to a fracture, which ended in departure of the crown from the skirt.
The picture of the broken piston crown shows that the nut/s have mostly located at the area on the lower side of the picture which is 90 degrees from the centerline of the gudgeon pins which would have caused the piston to behave as described above.

The fact that the fracture has stayed above the centre of the gudgeon pin has kept the skirt attached to the con rod and gudgeon pin.
I do believe that if it had run much longer the skirt would probably be in the sump as you described RonnieJoe.

Jim Twaddle
Biggar, Scotland

[rjschoolcraft]

There is simply not enough clearance for piston to rock enough to do what you describe without breaking the skirts first...

My opinion, of course. ;)

[Dvldog 8793]

Howdy
Scoring pictures would seem to be indicative of the skirt being pushed into the cylinder wall??? Possible that the FOD was moved from each cylinder creating the scoring in each as it seems that the score lines up with the skirt. Also I think Twaddle might be on the right track, the piston top was stressed and it broke in the weakest area... this could have been aggrevated by the above mentioned "over-cleaning" of the ring grooves...It would be intersting to see what the other pistons look like under CLOSE scrutiny...
L8r
Conley

[rjschoolcraft]

No doubt the pistons were pushed into the walls...

[rlvelin]

I would tend to agree on the piston rocking theory for the demise of piston #6. If the rings had been tight and a stress riser was present in the oil control ring groove there should be some indication of early fatigue cracking. There would be beach marks, smooth shiny metal with concentric darker rings indicating propagation and arrestment of the crack for every revolution of the engine). Every time the piston would go down the crack would spread and when it would come up again the previously joined metal would smash together smoothing and shining it and the crack would stop. This would be repeated on the next cycle until there was not enough crossectional area left in the piston to support the friction force from the tight rings and it would fracture the rest of the way all at once.

From the pics it looks like a pure brittle fracture (dull colored and rough fracture surface with many little peaks and valleys) suggesting it happened all at once from the tension force due to bending put on the side of the piston opposite the side where FOD was supporting it. Gravity would tend to force any FOD to the outside of the bore and perpendicular to the wrist pin.

I also initially thought that if the piston was being "bent" in the bore it would have broken the skirt off. However, the skirts look pretty beefed up and may actually have more crossectional area than the path the fracture took, which very well could have started at a stress riser in the oil control ring groove. Also since the fracture is not straight across the piston it failed partially in shear rather than only tension and the shear strength of most metals (maybe all?) is lower (usually quite a bit lower) than the ultimate tensile strength. Additionally the largest moment (bending force determinent) from the rocking theory would be occurring horizontally out from near the top of the wrist pin.

If you were to break piston #3 the rest of the way where #6 broke you would probably find beach marks on that one, do to the fact that it hasn't broke yet and the piece of FOD in that cylinder is smaller than a nut and would not generate a larg enough moment on the piston when it came to TDC to break it all at once. The object in bore #6 definetely looks like some hardened piece of an engine component that has been rattling around probably since the first time you fired the thing up. You aren't missing any parts of the valves, valve stems, or pre-cups are you?

Just my humble oppinion based on what I see in the pics and read in the posts. I know there are those out there on this board that know much more than I and I hope they will chime if I'm way off base here and with what they know. In any case I hope you can resolve this and get back on the road.

[dieseldummy]

Last time I had a piston "rock" in the bore of a 6.5 it cracked the cylinder wall like RJ mentions. The piston it's self was fine other than the valve sticking out of the top... The rod and cyl wall took most all of the flex that occured. The to tight of ring gap theory make the most sense of anything I've heard yet. It could have been a case of too much to do and not enough time to do it when the engine was assembeled in Georgia...

[moondoggie]

Good Day!

"6.9L and 7.3L engines reguire SCA's to prevent coolant cavitation, caused by thin cylinder walls." Please correct if needed, but I thought this was due to those engines having sleeved cylinders instead of cast-in cylinders. (I have NO personal knowledge if this (liners) is true or not.) The theory I heard was, the liners can move a tiny amount, & the firing event allows them to ring, just like an ultrasonic cleaner, which causes the cavitation, which then causes corrosion & ultimately failure of the cylinder. My impression was ALL diesel engines with liners need the coolant additive.

Please correct me if this is wrong - it'll sure be FAR from the 1st time. :confused:

Blessings!

[DmaxMaverick]

Yes, and no. I'm not certain which applies, but depending on year, series and application, both sleeved and cast were used. In any case, an SCA was/is needed. Ironically, Detroit sleeved motors don't require additives, and Detroit designed the GM Diesels, until the Duramax.

As a side note, a common belief was the GM Diesel engines were a "converted" gas engine. Not true. The engines were designed by Detroit as a full-blooded Diesel engine, but shared displacement and configuration due to the applications intended. The 6.9L and 7.3L, on the other hand, were "converted" from IH/Navistar gassers, which may contribute to some of their characteristics, including cavitation. This may also attribute to their relative inefficiency, as compared to the GM counterparts.

[arveetek]

Originally posted by moondoggie:
Good Day!

"6.9L and 7.3L engines reguire SCA's to prevent coolant cavitation, caused by thin cylinder walls." Please correct if needed, but I thought this was due to those engines having sleeved cylinders instead of cast-in cylinders. Sorry, this info is not correct. All 6.9L and 7.3L Navistar engines had cast-in machined cylinders, no wet sleeves. The pre-Power Stroke 7.3L engines were particularly prone to cavitation, since they made from a bored-out 6.9L block, resulting in really thin cylinder walls. It's fairly common to find a 7.3L that has been sleeved to repair holes in the cylinder walls, though.

I don't believe the Power Stroke engines are near as prone to cavitation, however, due to a beefier block.

You are correct, though, in that diesel engines with wet sleeves have a bigger problem with cavitation, due to the cylinder walls being able to move a tiny amount.

Casey

[eracers999]

Cavitation :

Cavitation exists in all diesel engines. Extremly small air bubbles that are created or that form along side the cylinder wall when the engine is running, they jiggle or etch when the engine is running along side the wall or liner. When these miroscopic or small air bubbles become acidic they will literally bore a hole in a cyl wall. The use of SCA's will counter acidicy. The material that the block or wall is made of will determine how fast or if at all this happens. I believe D Max correctly stated that it is a none issue in the GM blocks (refering to the use of SCA"s). SCA's are needed for the other things like head gaskets, radiator and heater core. Acidicy is what what turns the cores blossomed with white crap and rotts the head gaskets to the core .

Kent

[NH2112]

It's not air bubbles that are present during cavitation, at least not in the situation WRT 6.9l/7.3l IDI diesels. Rather, it's vapor bubbles from the coolant reaching its boiling point, but the vapor bubbles aren't able to get any bigger (like they can in a pot of water set to boil) due to the pressure in the cooling system. Air bubbles are only present when the cavitation results from an oil pump, water pump, etc sucking air, and I'm not entirely sure that this really is cavitation. It's just an oil/air or water/air mix.

From what I've been told, the problem with the 7.3l is as the piston moves in the bore the middle of the stroke gets hotter than the upper and lower ends because the piston "spends more time" in the middle than the ends. This creates hotter spots in the middle, which means the coolant in those areas is hotter and forms vapor bubbles (cavitates.) These vapor bubbles implode almost immediately under the pressure of the cooling system, which causes minute shockwaves as the surrounding coolant rushes in to fill the space. These shockwaves, over time, are what pits the cylinder walls.


I'll have some good pics of a destroyed engine soon! :D I know what caused it, though LOL

[eracers999]

NH2112:
Hmm, dont care what you want to call it, air bubbles, vapor bubbles, nor do i care to debate how they are created. The fact is that they are present and the only way they can make or create electrolisys is if the coolant becomes acidic. There is a simple test for this and 10 out of 10 times you will find the same result. If the cooling system was monitored from new, using a simple strip test to monitor SCA's or the lack of, and adjusted properly, the engine will not have a problem, 7.3 6.9 dont care what it is. It is basic fish tank theory as i call it.

Kent

[Kennedy]

When my '96 split the blocck/crank, I had one piston split, BUT that slug hit the counterweight and the rod was bent etc. Most of the pistons had marks where they hit he head and or valves...

No scoring and there was still crosshatch in most of the bore.

[patrick m.]

every "V" style engine i have seen with debris damage in a cylinder, broke a skirt and the cylinder bore.

I take that back, i repaired one late 90's 5.7L Chevy that just had the bore cracked....it had skirtless pistons.

Also had a freind who left a shop rag in a cylinder while replacing head gaskets on a 305. bent both valves, cracked the bore, broke the piston outside skirt, and bent the rod.


I have to believe something else started the crown crack, but i will buy the nut finished it

[rjschoolcraft]

As I stated previously, an initiation site appears to be visible (the "beach marks" that were referred to. John, any better pictures available?

[john8662]

Thanks for the input guys! I'm trying to view this from multiple ways.

I am curious why the piston cracked the way it did. My first impression was that no matter what, the nut(s) caused it and that I shouldn't think about it any more. But, from the discussion in this thread, I'm willing to consider that other things may be at work. I also througt that, geez 6.5 pistons (in general whether they be Mahle or not) are really cheesy pistons, especially in the area where mine broke. But, one thing I can see is that the weak point of the piston may be where I have breakage, as the skirt (or what little skirt these pistons have in general) is pretty beefy.

I've heard from lots of people about how an engine injested something and it beat the living crap out of the top of the piston and the head and or valves, but never split like that.

Ronnie, I have MUCH better quality pictures that I can post, what exactly would you like to take a closer look at?

I can also post a picture of how the other piston cracked from the unknown piece of FOD for comparison if interested (bottom side).

[rjschoolcraft]

In the photo where you are holding the rod looking down at the bottom part of the two-piece piston, there is a shiny area at the right, near where the wrist pin is. I'd like a closer look at that.

Thanks.

By the way, Cummins uses a two-piece, articulated piston...they achieve the two-piece objective with a different approach, though. :D

[restoguy]

While we're on the subject of engines ingesting things I'd like to throw something out there. A local mechanic with a pretty good reputation told me that he has, several times, fed broken glow plugs though 6.2L's! He said that when they come in with the tip broken off he just starts 'em up, closes his eyes, grits his teeth and gives it a little throttle until they find their way into the exhaust. He did tell me that if they don't leave pretty quickly, he stops and pulls the head. He never mentioned having any trouble with damage from this procedure. Not that I'm recommending any such thing. Just wonder what others experience might be with the same kind of situation.

[john8662]

Here is a closer pic that I had already shot of the bottom section of the piston still attached to the rod.

Here (http://members.cox.net/acmdsl/www/sub/cat/no6-bottom-big.jpg)

Heard of that glow plug removal method too, never seen it done though (don't want to be around when it's done).

[NH2112]

Coolant vapor bubbles that cause cavitation have absolutely nothing to do with electrolysis. Air bubbles and vapor bubbles aren't the same thing, unless you happen to be talking about vapor bubbles in liquid air. An engine that has its coolant pH monitored by test strips can still suffer from cavitation damage to cylinder walls, since the 2 have no connection.