I recently picked up an 86 F-350 6.9 IDI with about 140k miles, and I am quite unsure what caused this kind of piston damage. The damage is on 7 of the 8 pistons, and two of them are cracked right in the middle out of the 8. I would presume this engine has less than 50k on a rebuild, as the pistons are 20 over, the bores don't have any major damage, and that the bottom end is in such decent shape. Checked glow plugs, no tips have broken off, and the heads look very good. I also removed the pre-cups before i took the photos. Any ideas?

 

Did you make all those little dings in the top of that one piston?

 

These pistons are only 2 of the 8 that came out of the engine shown, this is what i was greeted with when i pulled heads off. I hadn't rebuilt this engine, but at some point, somebody definitely has. I got the truck with two engines in the bed, one was in the truck (the one that is shown) and running back in 2009 i believe.

 

You did not comment on the little ding marks on the top of that one piston. If you did not do that, then something has gone into that cylinder when it was running. Possibly a g!owplug tip?. The rest of it looks like a blown headgasket and the overheating that goes along with it when the problem is ignored.

 

Sorry for the lack of clarity. I removed the glowplugs and none of them had broken tips, but what I really do not understand is how the cylinder bores look really good considering the circumstances, and that there is no major head damage. I tend to believe that the pistons were installed with a hammer and a steel rod of some sort when the engine was being put back together.

 

The Ding marks in the piston are from the prechamber cup that fell out and was demolished by the piston. looks like you are missing a couple prechamber cups.

The Cups fall out primarily from ether use with glow plugs. Or the Re-builder was a hack and never staked the pre chamber cups in place.

This how the 7.3L head should look.

 

I actually removed those pre-chamber cups before i took the photo.

 

Well that is good news. Then the engine digested some debris. And chewed up the piston.


Pull the valves on that cylinder and check for bending (imprint of intake on piston) Check the rod for straightness in that cylinder. If cylinder wall is ok.
Give the wall a quick cross hatch up with a flex hone (If it is ok) replace piston and rings with new and you are good to go.

 

Well, thanks for the guidance. In the near future i will pull some valves and inspect for issues.

 

Sure looks like the valves and pistons came together. Are the dings on the tops of the pistons the same size as the valves and do they line up that way on the assembled engine?

 

"WAG Alert": "Rebuilder" installed a damaged piston. If his first .020 rebuild had a problem, maybe he cleaned it all up, took out the debris and decided that those dings in the piston weren't so bad.

Maybe.

 

The small dings are not in line with the valves, but the one big round indentation did. I compared the partial circle indentation (the right piston shown was the only one with valve indentation) with an intake valve from a 300 (i had a head disassembled), and the valve was slightly smaller than the circular indentation on the piston.

 

Some how debris got in the engine, who knows what it was we will never know it is long gone, no sense worrying about it. What ever it was prevented the the intake valve from fully closing causing the impact damage on the piston. (Circled below). Both intake and exhaust valves need to be checked to see if they are straight as well as that rod. If the cylinder wall is not gouged it can be honed with a flex hone recross-hatched (the minor surface rust will blow off when honed) A new piston put on the rod it's bearing replaced new rings installed and it will be good to go. I've seen far worse single damaged pistons replaced and then the engine go on for hundreds of thousands of miles in a diesel.

If multiple pistons are damaged in the same manner the item in question traveled between bores , it is not uncommon for this to happen, something gets ingested in to one bore pounded around a bit then spit back out the intake valve and travel to another bore along the intake, only to repeat this until it is beat down small enough to be ejected out one of the the exhaust valves. (remember the exhaust always has less lift than the intake so what goes in through the intake may not get ejected out the exhaust.

 

I would disassemble those heads to see if they are usable. That engine was not treated well at all. I would also clean those cylinders and look at them closely, because the one pictured does not look good at all.

 

So maybe the previous owner had a glow plug tip bust off on him, either it got chewed up or he fished it out. Replaced that plug with one from one of the old motors in hopes that it didn't get banged up so bad, and just ran it as long as he could?