From talking with several reputable engine builders they seem to have a common theme that cranks with stock or tight clearances seem to break more than ones that are set up loose. Both if the ones that I have had break are stock to near stock clearances. Thus time I willing go back with my original build specs. About a thousandth over clearances.
Broken Crankshaft Alt Fire Cam
- Thread starter S Phinney
- Start date
Mark isn't kidding about the crazy vibration on the engine dyno at that rpm. Its almost scary sounding.
I'm just saying that is a common theme. It really doesn't mean much but it was relative enough to notice a trend.
True but can't imagine if it has the symptoms it's not going to end up being a crank.
Has anyone seen a trend with blocks that are half filled or filled vs a normal Blick on breaks?
All this talk about breaking cranks with alt fire cams is pretty depressing. I bought a brand new lb7 crank fir my next build and to be honest not really sure if I even want to take the chance of doing it all over again. 
All this talk about breaking cranks with alt fire cams is pretty depressing. I bought a brand new lb7 crank fir my next build and to be honest not really sure if I even want to take the chance of doing it all over again.
X2
How much cash do you think it would take to get this done? If we could peg this it would help the whole duramax community out. I have no problem throwing some money at it and I'm sure others would as well if we could get this figured out! As far as the 1600 to 1800 shake my truck drones at 1703 to 1805 those 100 ish odd rpms drive me nuts and give me headaches on long drives but that's right where 65-75 mph is my crank flexing what's causing the drone? Also makes me wonder if the highway miles in that rpm range are hurting it any?
At the chemical plant that I work, we have these vibration analysis boxes that we check vibration on equipment. I am just learning how to use these things but one thing that I have learned is all metal has a natural freqency that will "ring up". When we are having trouble out of a steam turbine, centrifugal compressor, centrifuge, electric motor, etc we go out and do what we call a "bump test" to check the housing natural freqency and mounting baseplate natural frequency. Most of the time when we cant keep bearings in a machine or they self destruct, we find that the natural frequency of the housing or mounting base matches the run speed of the equipment causing vibration inside the equipment and the reliability is greatly reduced.
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Same kind of thing is required here, but taking the measurement is a real PITA on a rotating crank. You need displacement sensing in probably 3 axes (at least) with non-contact sensors....all correlated to a rotational position. The data stacks will be large.
I'm sure one could datalog some really simple max displacement measurements in single axes to get started. I'm sure we'd see what was going on. To gather data that usefully plugs into FEA is a big jump up from there in sophistication, IMO.