LLY: Broken piston skirt

bimmer95

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Mar 15, 2019
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So im looking for some input.my lb7 has 270k miles on the truck.the motor has approx 80k on a rebuild with delipped stock pistons,lbz rods,balanced rotating assembly,arp main an head studs.it also has 45 over sac injectors and 366sxe turbo.anyway it developed a knock after a full throttle tune 3 pull from about 30 to 70 mph.definately bottom end an not injector knock.so I tore it down an the cyl 5 piston skirt is broken all the way up to the oil ring as u can see in the pic.im wondering if anybody can help me out as far as what caused it?now I know these things bend rods all the time but this is cyl 5 where the crank doesn't even come close to the little tab on the skirt and the rod isn't visibly bent.also I might just throw in that injector 5 was the worst one as far as balance rates I'm talking before this all happened..around -4.5 I think the rest around or under 3.so could a bad injector do this?or is it just coincidence?I'm kind of wanting an explanation before I put it back together so I can avoid it happening again..ive rebuilt several duramaxs in my time but this I haven't really heard much of..??also note the crack by the wrist pin


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Chevy1925

don't know sh!t about IFS
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Oct 21, 2009
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only way thats happening is either a bent rod or broke crank but the crank definitely hit the piston. should have verified piston protrusion (or lack there of) before you pulled the piston/rod assembly out.
 

Fingers

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Apr 1, 2008
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Not enough pictures to be sure about anything.

Obviously the crack had been growing for a while. The pisser cutout has an old dark crack that goes bright when it gave up the ghost.

The oil ring too shows an old/new transition going from the ring into the oil gallery.

In short, I think the piston may have been flawed in the first place. Just not in a critical place and it has taken a while to fail.
 

bimmer95

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Mar 15, 2019
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Thanks for the replies,a couple more pics as well.they are delipped stock pistons.i thought about it once about checking the protrusion but dumb me in my eagerness to take it apart totally forgot it..it crossed my mind that it may be just heat fatigue as this engine has seen a crazy amount of heat in it's time..but not sure if that's even a thing with this material.


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bimmer95

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Mar 15, 2019
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I think I'll take the rods to the machine shop and have them check them just to give me some closure lol..the last pic is of a completely different hole an I think it may be from a ring gap closing and tearing up the bore?

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LBZ

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Were they new delipped or previously enjoyed delipped pistons?
 

2004LB7

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Dec 15, 2010
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How's the piece that broke out? Does it have any marks or indication it made contact with anything, crank?

Kind of hard to see but it almost looks to me that the fracture looks like it was from pulling out or away from the piston. Look that the area right at the ring and to the right. That to me is an outward break. I'm with James on the crank contact
 

bimmer95

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Mar 15, 2019
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How's the piece that broke out? Does it have any marks or indication it made contact with anything, crank?

Kind of hard to see but it almost looks to me that the fracture looks like it was from pulling out or away from the piston. Look that the area right at the ring and to the right. That to me is an outward break. I'm with James on the crank contact
Ahh just a bunch of tiny little pieces..it must've really got butchered up by the rotating assembly on the way down.yea im just not sure..if it was next to one of the thicker counter weights I would definitely say the rod is just twisted or compressed a bit.but it would take a pretty maimed rod on cyl 5.an yet mabe I'm just seeing it wrong..

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JoshH

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I’m having a hard time figuring how a crank will touch the piston skirt in the area where this piston is damaged. Maybe I’m just not thinking about it right. I can see if it had damage at the pin boss, but how would a crank touch the piston in that area?
 

ikeG

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Apr 19, 2011
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I’m having a hard time figuring how a crank will touch the piston skirt in the area where this piston is damaged. Maybe I’m just not thinking about it right. I can see if it had damage at the pin boss, but how would a crank touch the piston in that area?
Yea, if a rod is bent the pin boss will kiss the crank counterweight.


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juddski88

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From the pics you posted it certainly looks like there's some markings on 1 or more pistons in the wrist pin boss area. Which crank was used? Just curious. There seems to be room for multiple things wrong in this build.
 

bimmer95

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Morgantown Pennsylvania
From the pics you posted it certainly looks like there's some markings on 1 or more pistons in the wrist pin boss area. Which crank was used? Just curious. There seems to be room for multiple things wrong in this build.
Lb7 crank.im assuming the pistons were matched to weigh the same that's why there's material missing here an there..the rods as well.

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juddski88

Freedom Diesel
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Lb7 crank.im assuming the pistons were matched to weigh the same that's why there's material missing here an there..the rods as well.

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The marks I'm seeing don't appear man-made. Any marks on the counterweights? Next up is measuring the bore diameter from top to bottom, in 4 quadrants.
 

bimmer95

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Mar 15, 2019
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Morgantown Pennsylvania
The marks I'm seeing don't appear man-made. Any marks on the counterweights? Next up is measuring the bore diameter from top to bottom, in 4 quadrants.
Yea well I don't have the tools to measure the bore super accurately right now..mabe I'll get the machine guy to look at it..i can't find any abnormal marks on the crank..this is the counter weight next to the number 5 rod..not going to hit unless the rod is pretty majorly compressed an no marks..


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bimmer95

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Mar 15, 2019
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Check the other pistons in the area highlighted in the picture. I suspect that is the true crack origin.



I checked all the rest an none of the others are cracked..but o well it won't happen again,I found a great deal on some r&r rod,diamond forged piston assemblys(I know it's overkill but hey u can't be too picky if u want a good bargain) so I'll be boring it 40 over and using those..

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