Sorry for the long silence on this thread. I've been working on and off on the Monty, but I'm not nearly as good as StudebakerHawk or Larry in keeping up with my progress (or the lack of) as well as posting the photos.

The original blue smoke was from oil sizzling on the hot exhaust pipes. We had some bad weather here for some time, so that delayed my work. Eventually I was able to find a place in my friends farm barn to undertake this adventure. I've discovered that the weird brackets on front passenger and rear driver side of the engine are the built-in attachment points for the engine lift. Very convenient. After proper aerobics exercise (by which i mean crawling under the car about 100 times to disconnect everything), I was able to pull the engine. It came out easier that I expected, if you don't count the two (insert expletive here) bolts that are on the very top of transmission right up against the firewall and barely below the coolant cross over pipe.

One other thing to note for future adventurers - on 6G74 engine the torque converter access plate is on the driver side of the engine opposite of the starter. The only way to get the six TC bolts out is the 17mm short socket and a 3/8" flex handle. You can't get a ratchet in there and bolts are recessed far enough that a box end wrench does not fit. Adding any sort of extensions to the socket in order to move the ratchet out of the way, misalignes the socket and the bolt. You can't really use a flex extension on the socket ether, as you don't have much room to maneuver and again you risk poor contact of the socket and bold and as a result you run a risk of stripping the head of the TC bolts. Flex handle works the best.

As soon as drive plate came out, I knew why oil was pouring out of the back of the engine - rear main seal was pushed out completely. It was old and dried up and I was in heaven - my misery is solved. It was not really surprising, since the rear main seal was still original from the day the engine was assembled in Japan. Trying to track down the misfire on #5 I also discovered that the intake valves (visible through intake now) were covered in oil. I pulled the head and removed the valve springs. To my surprise it turned out that I somehow messed up on of the valve seals on the #5 cylinder intake valve when I replaced them 9 month ago. That gave me hope that this is where the misfire was coming from. Just in case and since I had everything out at this point, I checked the main bearings and couple rod bearings. I was impressed with the fact that all of them were perfectly within spec (about 0.035-0.040mm). With that knowledge and full of optimism, I replaced rear main seal, front oil pump seal, cam seals and few other gaskets. I decided not to remove the heads because all looked good from inside the crank case and the gaskets were not leaking. Slapped it all back together, dropped the engine back into its place and I was ready for my victory lap....

F***ing misfire on #5 again....... total disappointment and defeat.

I decided to follow proper diagnostics - check for ignition, fuel and compression. Swapped neighboring coils - still misfire on #5. On to compression - all cylinders are good, #5 is "0". I am not kidding - zero. That's when I went - Oops... Did i screw up timing somewhere during assembly and managed to bend a valve. No problem, I can figure that out and in the worst case scenario pull the offending head and replace the valve. I decided to run a leak down test to see which valves I messed up. Put the leak down tester on the cylinder and I'm ready to listen for air coming out of intake, except it is coming form the PCV hose and from the dipstick. That means its leaking into the crank case an it is a major problem.

Instead of pulling passenger side head, I pulled the engine out again (this time at home outdoors). You know how they say practice makes it good... I was able to do second round in one morning instead of two days as I did during the first round. Got the engine on the stand again and pulled the passenger side head only to discover a hole in the #5 piston

Major major problem. The cylinder is shot. The chunk of aluminum had broke off from the edge above the ring, bounced around in there and got pulverized completely. Some of it deposited on the bottom of the head. The walls of the cylinder are slightly scored, likely not a deep gauge, just minor scratches. At this point, we are in for a full rebuild. I pulled all pistons out. 1-4 look great - no wear of any sort, just proper discoloration from age. Piston rings all move freely. #6 piston is good too, but it started to score in a funny way, similar to #5. The wear in not on the piston skirt where you'd expect it, but on all four sides at 45 degrees from the piston pin. Can't figure out how and why the piston would wear like that. Connecting rods are moving freely, no binding, not bent, no excessive play, nothing out of the ordinary. Cylinders 1-4 still have perfectly nice crosshatch on the walls. #6 has couple of vertical lines where the piston started to wear. No wear at all on crankshaft mains or on connecting rod bearings. All bearings are all still good with no areas where the friction material is missing.

Anyhow, I'm looking for a shop to clean and bore out the damage on #5 and #6 cylinders. Unfortunately, so far I was able to find only one good reputable shop in the area that will do the work. But these guys do full long block rebuild - everything top to bottom, pistons, bearings, oil and water pump, valve springs, heads, etc. All new or machined to specs, properly balanced and tested. 3 year warranty on the engine. The only problem is the $$$. It is much much more than I was hoping to spend. I have not committed to it yet, still waiting for a call back from couple more places hoping to get away with just a short block rebuild and doing most of the assembly myself.


Rear main seal pushed out

Torque converter access hole. Not much space to work around there.

Plastigauge on main bearing. All other were in the same range.

Houston we have a problem....

Scoring on the walls of the #5 cylinder

Neighboring #4 cylinder walls still looking good after 250k miles.

And this is why were are were we are....

 

Hunter.......Some great pics there of the "situation".....yep, you've got a "real situation" going on there. What do you think was the point of failure in that piston? Did all that damage start by a cracked ring which ended up on the top of the piston? Its a wonder you did not hear some major screeeeeeching/squeeling/crazy scraping noises going on what she "blew".

Maybe you will want to "do it on the cheap"......just hone out the cylinder walls and just order one or two pistons/rings and put back together? Another option....maybe find a good used engine to replace the old one?

 

The lack of clarity on what caused the failure is my biggest concern. I can't figure it out. The compression rings on #5 piston are completely intact. They are stuck due to debris jamming them up. I think the oil ring failure is secondary to the primary piston disintegration. Curiously, like I said - No 6 piston and bore are starting to show similar wear pattern to the #5. It is at 45 degrees from the pin (photos). Those rings are also free and moving around too. They are starting to catch a little when the gap crosses the most excessive wear areas. If anyone has ANY theories, please share them. I'd be happy to hear any explanation why both #5 and #6 cylinders have this happening, but not any other.

The cooling system never malfunctioned, engine never run hot or out of oil. If you can believe it, i ran original water pump on it all this time. Once I pulled the engine and was ready to replace the water pump, I thought I'd take a look inside to see how much wear is there. Honestly it looks as good as new. No wear or corrosion on the impeller at all. Spins freely too. I also checked the oil pump to see if perhaps it wore out and not supplying sufficient oil to the engine. Nope, no luck, it is completely within specs on all measurements.

As far as doing it on the cheap. It is one of the options I'm exploring. The problem is - it sounds like good shops will not do just one of two cylinders. The shops that may do it do not come with good recommendations. My mechanic friend said that just repairing two cylinders and installing two new pistons may be risky. It is not going to be a matching balanced set of cylinders and it may not last long. The way I think is - what if the other pistons are ready to give up the ghost? I mean, I'm getting really good at pulling that engine out, but I'm not too exited to have to do it again
As an temporary viable alternative, I'm exploring a possibility of a used engine with lower mileage. Got couple of leads. Waiting for an answers. I figured if I can drop an engine into it and drive for now, good enough. I can probably get another 100k out of it. At the same time, if I win a lottery, I can do a proper rebuild on the engine I have now


No 6 cylinder wear


Water pump after 250k miles. Amazing.

 

Hunter.....Ummmmmmm.......This is a very perplexing riddle. "The Mystery of the Blown No. 5 Monty Piston". The external piston damage on No. 5, and beginning to show on No. 6 has me wondering......would it be possible for the rear of the crankshaft to have a totally shot bearing which was causing some minor "wobble" in the crankshaft under load......and that slight wobble translated upward to cause some odd vibrating of the piston, maybe even extreme up/down wear at about 45 degrees from the pin. Or, could it be a shot piston pin causing the "shuttering" of the piston......but then why would this be happening on the opposite cylinder No. 6?

But now Im thinking about that blown rear crank seal.......which makes me go back to a very slight wobble in the rear section of the crankshaft. Maybe the blown rear crank seal and the blown-out piston were caused by the same gremlin. But I thought you said all the crank bearings were good........if so, can something have become out of balance or out of round from tolerances on the rear of the crankshaft?
Just some wild guesses/ideas. Hope an expert can shed some light on this situation.

 

All four main bearings were nice and had even wear. I think the rear main blowing our was a result of piston failing and combustion gasses escaping into the crank case, pushing out the seal. The photo of the plastigauge I posted is from the #4 main bearing (closest to the seal). I'm not ruling out anything, but no visual abnormalities anywhere on the crankshaft. Bearings on all connecting rods are good too - no excessive wear.
Keep those ideas coming. You know I don't ask unless I'm going to stump everyone with my problem.

View of connecting rod bearing on #6 piston

 

Interesting......scratching all my hair out on this one. Since nothing fell onto the piston from above (spark plug breakage, valve, etc.), it seems the hole in the top of the piston is due to melting? And it looks like excessive wear/tear along the piston sides (about 45 degree offset as you have shown). So, I guess the piston was creating so much friction and heat that the piston started to melt on that side. Im not sure how the oil galleries work on the Monty to properly lube the sides of the piston, but could something have clogged-up in that area to cause a lack of lube on the outer piston surface?

How about a clogged water gallery that blocked coolant flow to the rear of the engine?

"If" you did merely hone the cylinder and slap in a new piston, would the same thing happen again?

Would like to hear what a machine shop has to say about it.

I recall watching an episode on Youtube of that New York machine shop who specializes in rebuilding/restoring old classic big block Mopars.....440s, etc. He had a big 440 brought in that had a piston utterly destroyed, similar to yours. Turned out that a mechanic had dropped a small nut down the intake manifold while putting a new carb on, and it got sucked down into the running engine and was like a hand granade going off.......heart breaking.

 

Interesting theory about the oil gallery. It is the only thing that might be coming close to any explanations. At the same time, wouldn't you expect some uneven wear/damage on the bearings around that area on the crankshaft? I wonder if there is a way to test the little oil spray nozzle on the connecting rod. I'll have to look into that.
Water passages are all clear around both #5 and #6 cylinders.

 

Lack of oil to No. 5 and No. 6 pistons was one logical potential cause......wondering if you can verify the oil flow somehow. Does the oil pump pump oil up and from the front pistons and down to the rear pistons? Who knows, maybe over the years it was beginning to clog up. Thats one reason I have been using 1-2 Qts of Marvel Mystery Oil at oil changes on these higher mileage Montys.....I belive thinning down the oil a bit may help clean out those arteries and help avoid these massive strokes/heart attacks.

Maybe the lower crankshaft naturally gets a better flow of oil since it sits so close to the oil pan??? Maybe gets more "splash" action?

With the water passages being clear for #5 and #6, then the overheating would not be from a lack of coolant.......but once again points more toward lack of oil.
Will be great to get a confirmed answer to the riddle.

 

Well, I have some good new for a change. I got to connect with some folks on another forum I visit. Many of them have experience with engine building and work and were very helpful in identifying causes of my engine failure. One of guys was in fact an engineer at a Detroit lab that would purposefully run engines till they self destructed and do failures analysis on them. Once I posted the photos of the damage, reply was - "I know exactly what happened here. I've seen it before".

What many of them said, is they suspect that cylinder #5 and to some extent #6 were running very lean. If you think about it, our engine is port injected. I suspect that computer calculates an average open time for all three injectors on each bank based on the feedback from the O2 sensors. Even though each injector fires individually, the pulse length is probably the same for 1-3-5 and for 2-4-6 injectors (I'll have to look more into that to confirm). If one of the injectors is not completely bad, but puts out significantly less fuel then the other two on the same bank, that cylinder will run leaner, even thought the entire bank data comes back as OK or with some minor positive fuel trim. As long as overall fuel trim on the bank is below the threshold to set CEL, you have no idea it's happening. Anyhow, this lean condition on one or two cylinder caused localized overheating. With pistons 5 and 6 being in the back of the engine, they are already at a disadvantage as far as cooling capacity goes. Running hotter than desired, caused detonation in #5 cylinder. This detonation is not the same as regular pinning that a knock sensor would pick up and adjust ignition timing. It has something to do with spontaneous detonation that happens in the combustion chamber at the same time as spark plug ignites fuel mixture. There is some sort of condition occurs where flame doesn't just propagate from spark plug throughout the combustion chamber, but instead another spot ignites at the same time and two flame front travel in opposite direction to each other. if this happens during the normal ignition event, computer can't detect it. Apparently with the overheated piston, a "logical" spot for this detonation spot to start is the edge of the piston. It is the hottest area and kaboom!!! Flame fronts collide and after multiple undetected events a small part at the edge of the piston breaks off.

Once that top land area is open, all combustion gasses escape past top compression ring, acting like a blow torch. This melts all the material on the side of the piston in one take. All combustion gasses are now going into the crankcase and blow out a rear main seal in the process. So, the good news is, all that scary damage on the #5 was not caused by long term abrasion and wear, but by a single fire torch event. Quite literally - a single combustion cycle that went bad. As far as funky wear pattern seen on the #6 cylinder, it is also related to that suspected localized overheating. Apparently the pin area in the piston is made from different material than the thrust face and the skirt (I had no idea about this). This results in uneven heat expansion of the piston, especially under abnormal heat condition. This would force the four "corners" of the pin seat area of the piston into more contact with the sides of the cylinder, causing even more heat build up.

These guys are also pretty confident that the engine can be repaired by just replacing the two damaged pistons and re-honing the cylinder walls. They insist on NOT BORING OUT any cylinders. Just hone it, put in two new pistons and be done. I was finally able to get in touch with a good reputable shop nearby. Talked to the owner and he seem to be positive that based on my description of damage it is easily repairable. I'm taking the disassembled engine to him tomorrow, so that we can look it over and to confirm the repair procedure. He can order pistons and rings and fix the cylinder walls. I get to put it all back together. Sounds like a Win-Win situation to me. I'm also going to take my fuel injectors to another shop nearby so that they can do a flow test on them. I want to see how much difference is there between injectors on 5 and 6 vs the rest of them.

 

Hunter......Wow, very impressive write-up and effort to contact the experts who solved this riddle. Who needs a Youtube video when you describe that "double flame nuclear-hot pre-detination" in such detail that I can visualize it perfectly. It did not even dawn on me that the rear injectors could be squirting a super lean mixture.......but that lean mixture would defiantly make the mercury rise.

Excellent news that you can just re-hone and slap in a couple new pistons/rings and be off to tha races. Im thinking maybe you will want to also install a couple new injectors also, or be able to clean them and test their flow before re-using.

But this leads to another question......Do the injectors typically go bad on the rear of the engine (No. 5 & No. 6) first? Is it due to the little extra bit of heat back there?
(Maybe 4 ounces of good ole Marvel Mystery Oil in every tank of gas would help keep those injectors clean?)

Any way to tweek those rear injectors to squirt a wee bit on the rich side, so with time they would normalize instead of squirting lean?

Super interesting story.........hope the rebuild of 5 & 6 goes well.

So, the old hillbillie idea of "just re-honing and slapping in 2 new pistons" may have been ok........EXCEPT the hillbillie would not have thought to replace the injectors, and the engine would have been destroyed again.