It's Official I have a Broken Crank

[LBZ]

Out of curiousity, did the allign boring get done before or after the install of the main studs?

Word on the street is that the LBZ/LMM cranks and blocks are built a little stronger but I don't know if this is 100% accurate. The majority breaking are LB7/LLY's, but this could be because there are more of them out there with more miles on them.

 

[MadMaxx61]

LOL, I thought alittle humor in the face of woe was in neat,

That was a nice touch.

Sorry it happened but I am happy you are sharing all the info you find as you are fixing it.

 

[tylerrustLB7]

This one sheared the pins.


http://www.duramaxdiesels.com/forum/showthread.php?t=15171

 

[wdino73]

I'm going to be honest like I posted, I didn't line-bore, I checked the bore and everything checked out and by the wear on the bearing I was right, I didn't change caps and used the same crank, I know some people will disagree but the proof is in the bearings.

 

[FMOS]

Got better photo's of the fracture surface on the crank?

I imagine the surfaces are pretty beat from rubbing together once they split though? Might not be enough "evidence" left in the appearance to tell much of anything.

 

[Burn Down]

Like mentioned above, post some better pics of the fracture. From what I can see in the pictures the crack started on the surface in the fillet. Can you get a better picture of the other fillets (the area where the bearing surface starts to transition into the un machined portion of the crank shaft)?

Did anyone do any prep work to the crank, polish or turned?

The crank looks to have been cracked for awhile the beach marks are very tight (slow) at the surface and slowly get wider (fast) as the crack grew into the center of the crank.

If you didn't prep it at all, then I would say you did nothing wrong assembly wise to make it break. The fillet was doomed from the factory. Wrong transition angle on the fillet, nitriding, to hot when machined? Who knows? Many things can cause surface cracks & I am no expert. You would have to send it in for failure analysis and thats spendy.

On a side note maybe this broken crank or others could have been caught if they were crack checked at a machine shop before assembly.

Still seems odd that they are breaking in pretty much the same place... Maybe I'm full of it?

I would like to see you take it to a machine shop familiar with the duramax and have them check the rest of the crank for cracks. Also they should be able to look at the fillets and make sure they are properly machined. Just a thought.

 

[LBZ]

I think good advice is to have the crank magnaflux inspected before assembly to ensure it is good. I plan to do this when I build my engine.

 

[Stingpuller]

Sorry to see your problems

I agree it's not in the studs or block. The bearings are good and it would have shown if not. Jeff

 

[TrentNell]

I think we all should chip in and send one of these cranks off for failure analysis , I would chip in a bit it would be worth i tif we got an answer . Even though I think we already have an anwser that aftermarket cranks may be worth investing in . Anyone know the cost on the Socal standard stroke crank ?

 

[maxaholic]

Excuse my ignorance since I've never been in the bottom end of a Duramax. But is there not a thrust bearing. And those are the main bearings in the middle aren't they.

 

[othrgrl]

Bearings aren't showing signs of the mains being a problem. Aren't these cranks twist forged? Could it be a stress facture that had been there since new and as the power was turned up on it the crack started getting bigger and bigger until it finally broke the rest of the way through?

I think we all should chip in and send one of these cranks off for failure analysis , I would chip in a bit it would be worth i tif we got an answer . Even though I think we already have an anwser that aftermarket cranks may be worth investing in . Anyone know the cost on the Socal standard stroke crank ?

I believe Guy has said that the cranks themselves are the same price. He started out by looking into getting a stock displacement billet crank made for strength. When the companies gave him bids on it he realized they were expensive enough that most people would want some performance to come with the price so he spec'd out a stroker crank and the price to have both manufactured was the same.

Excuse my ignorance since I've never been in the bottom end of a Duramax. But is there not a thrust bearing. And those are the main bearings in the middle aren't they.

They do have thrust bearings at the #5 main, but they are separate from the main bearings, unlike most gassers that have the thrust built into one of the mains. I ASSume he had the thrust bearings in.

 

[maxaholic]

Ok just checking.

 

[dmaxvaz]

i think we all should chip in and send one of these cranks off for failure analysis , i would chip in a bit it would be worth i tif we got an answer . Even though i think we already have an anwser that aftermarket cranks may be worth investing in . Anyone know the cost on the socal standard stroke crank ?

trent, do you know what #s you have on the front of your crank? If you have all 1s, its supposed to be better than the one with all 2s or the one with 1s and 2s.

 

[wdino73]

More Pics

Here is some more pics men

 

[fishsmith]

I had a stock dampner on mine if that matters. Mine was actually broke a little further back but not by far. It was broke up in the first rod.

 

[dmaxlover]

There seems to be no real pattern as to why the cranks are breaking. I ran a Fluidampr for almost 3 seasons of pulling with no issues.

 

[wdino73]

Well my years has been shitting anyway, I've got 11 screws and a plate that looks like a bike chain in my foot, I busted my heel in half. so way not bust my crank into, LOL