LB7 broke a crank

Harmo,ic balancers rarely fail on the duramax, I've yet to hear of one failing even on the million mile trucks. I know many have a hard time grasping it, but unfortunately it's the luck of the draw. It's a known problem, it does occur, and there is ZERO explanation for it. Theres been trucks go half million miles turn around and reuse those cranks in a race engine and put down a thousand hp with no crank problems. And then theres guys who have had them break with 6k miles on a NEW engine.

Theres many theories on it ranging from the firing order. GM has 1&2 firing back to back, and the theory is that it overstresses that throw as most break near the snout or around the rod journals for 1&2. So they now have alternate firing order camshafts. A few have still had crank failures though, but they were all used cranks that had been run with the stock firing order. But there has yet to have been a reported failure with a new crank and an alternate firing order cam that swaps 2&7 iirc. Another theory is the width of the crank across the rod throws, and the radius of it there.

Several have had the broken cranks examined, and the general consensus has been poor casting due to how the fractures appear to have porosity in many cases.

We DO know several things that have been proven to lead to crank failure. Poor tuning is the #1 killer. To much fuel down low with WAY to aggressive of a timing table. Particuliarly stacks of boxes like how we used to do it to get power. Another thing that was linked to ALOT of failures was running a fluidamper and turning the RPM's higher than stock. The fluidamper weighs I believe 11 lbs MORE than a stocker and all that weight is hanging out front of the geartrain. Another killer has been bad injectors. The ECM will balance all the cylinders at idle as best it can, but as soon as you touch the throttle, they all zero out, and fueling is even to all cylinders. If you have bad injectors, you can have dramatically different fueling from one cylinder to the next which invokes a nadty harmonic in the crank from the uneven firing pulses.

Ultimately it is a harmonic issue as the DURAMAX DOES have harmonic issues. It really isnt the smoothest running engines across the board as they have NUMEROUS RPM points that if you hold them at, they appear to run smooth, but will rattle and hum the trucks to pieces overtime. 650 RPM's is one known point that overtime the harmonics of the engine will wreak havoc with rattles and harmonic hums in the truck. Coincidentally, I believe this is where LML's idle at, and they seem to have the highest rate of broken cranks stock of any of them.

It's mostly all theories, but ultimately, it's just bad luck with the DURAMAX to have a crank break.

 

It's still uck of the draw. Theres HUNDREDS of broken DURAMAX cranks from guys running aftermarket balancers on engine with cirtually no run time. LML's seem to pop around the 150-200K mile mark, others are a gamble. As to harmonics, these same harmonics are present in engines running ATI balancers, it's just a natural harmonic in the engine. Wagler and I believe Callies are both working on forged cranks for the DURAMAX, and theres been some interesting stuff coming from that as well, but still not much.