Broken Crankshaft Count?

[Yellow Jacket]

New crank or good used that broke?
How much time on the build before it happened?

See, that's why I don't even say anything..... I run all used junk. It was a used lb7 crank that was mag checked...so there, that's why it broke. BTW, it was internally balanced also, had 55+ hooks and 2 pulling seasons on it.

I've seen a broken Sonny Bryant crank that started life as new & better material than stock... Just to help people understand why I chose to use a used $250 crank and have it "checked" vs starting with a new $1100 OEM one after freight and/or tax...just to have it break too, lol.

That's another big difference in pullers, most tend to build something with the understanding they are gonna be hard on it and not expecting it to last forever. I've paid $6400 for a class built turbo, (along with many others), hoping it'll last 25+ hooks and all 1 season. That's not even 100 Street miles and only 5 drag racing weekends for some racers...not many people in their right mind would do that!

The pulling engine I have tore down now, has 45 passes on it and already has .004 taper on the bore on a couple cylinders... Needless to say, I'm reringing the bitch and new bearings (which look great) and it'll be back in the truck for this season... Won't be any left in her after this year, if it makes it all year. It's a .060" over, full filled SoCal short block, that was running water filled Wagler heads until they cracked, and 2nd half of last season got solid Wagler heads and was completely waterless...

 

[Stingpuller]

Cranks

If it mags good I would rather have a used crank. It has been stressed and heat cycled. They all break at some point. Good to see others talking about problems (breakage) and understanding that it's part of the game. It does suck but such is life.

 

[DAVe3283]

If it mags good I would rather have a used crank. It has been stressed and heat cycled. They all break at some point. Good to see others talking about problems (breakage) and understanding that it's part of the game. It does suck but such is life.

That was my thought when I built my motor. My crank made it 300k miles and a bunch of 1/4 passes and checked out great. From what I have read, the odds of a new one breaking are about the same as my old one, so I figured I'd save the money.

I also re-used my oil pump for the same reason lol

 

[LBZ]

One or two maybe cases of new crank breaking with an AF (via heresay) but lots breaking with a standard FO and new or used crank and a few with AF and a used crank. And many of the used with an AF were either dedicated puller/drag or high hp.

Am I the only one seeing the connection here???

Mag only tells you about the condition of the surface. Doesn't tell you what shape the inside is like. I wouldn't rely on it solely. Nothing wrong with reusing a crank, but for my piece of I wouldn't risk it.



Sent from my D5803 using Tapatalk

 

[kidturbo]

Here is an off the wall question that's maybe been brought up before. What about a internally balanced crank and a new style fluidamper?

I know ATI is the go to setup because of the external counter weight issue, but lets reverse this for a second and say we ditch the weights then use the FD to it's full potential.

Back in the 90's I swore by them on high RPM race engines. In 2010 I stuck with a stock damper on the LLY boat engine due to aftermarket damper concerns. That engine is still spinning strong today BTW. To me the ATI is mostly a nice looking pulley with very limited vibration control abilities.

From the ATI website: The Super Damper is especially needed in high performance and modified trucks to help protect against harmful engine harmonics

I'll say we can pretty much scratch that statement off at this point... So could the FD setup be any worse at the job if everything was zero balanced?

 

[juddski88]

I posed that theory last year after doing a bunch of research on vibration theory. I don't like the limited ATI frequency dampening ability.

 

[Yellow Jacket]

Here is an off the wall question that's maybe been brought up before. What about a internally balanced crank and a new style fluidamper?

I know ATI is the go to setup because of the external counter weight issue, but lets reverse this for a second and say we ditch the weights then use the FD to it's full potential.

Back in the 90's I swore by them on high RPM race engines. In 2010 I stuck with a stock damper on the LLY boat engine due to aftermarket damper concerns. That engine is still spinning strong today BTW. To me the ATI is mostly a nice looking pulley with very limited vibration control abilities.

From the ATI website: The Super Damper is especially needed in high performance and modified trucks to help protect against harmful engine harmonics

I'll say we can pretty much scratch that statement off at this point... So could the FD setup be any worse at the job if everything was zero balanced?

I posed that theory last year after doing a bunch of research on vibration theory. I don't like the limited ATI frequency dampening ability.

Nearly every tractor pullerand semi puller uses a fluid style dampner if going aftermarket.... They've worked for years in other applications!

 

[kidturbo]

I posed that theory last year after doing a bunch of research on vibration theory. I don't like the limited ATI frequency dampening ability.

I found the same comments when reading up on both units. The frequency range coverage on the fluidamper is stated to be much wider due to its silicon gel design.

http://www.fluidampr.com/what/how-does-a-fluidampr-damper-work-55/

Reason I bring this up, Dave mentioned yesterday that while he and Nate were testing some tune changes a week before his crank failure, he noticed a "new hum" developed at a certain RPM. Because this a boat, engine noise is actually easier to hear and feel. The damper is also closer to your ear, right there behind you when driving. Nate commented that the hum was the damper doing it's job. But Dave had never noticed this hum before.

To me that statement could be a key to this specific failure. The hum wasn't present before, next weekend that crank breaks within a couple minutes of firing up. Was it actually a harmonics issue when the hum developed and it failed later, or was that hum a sign it was already broken?

From everything I've heard and read harmonics is likely a key to this fatigue, and the AF cam does change those somewhat. So maybe we should track the damper models ran on each failure, along with firing order and crank balance.

I tend look at other models like my inline 6 CDI Mercedes which you could stack wine glasses on the hood at 4500R's. Removing vibration harmonics from this 90deg V8 shouldn't be that hard since it has a "evenly spaced firing order" to start with. Besides that, I feel the rest is likely journal size as Sting has been researching. And probably not only the rods.. MB increased the mains on the inline 6 to remove vibration...

 

[juddski88]

I found the same comments when reading up on both units. The frequency range coverage on the fluidamper is stated to be much wider due to its silicon gel design.

http://www.fluidampr.com/what/how-does-a-fluidampr-damper-work-55/

Reason I bring this up, Dave mentioned yesterday that while he and Nate were testing some tune changes a week before his crank failure, he noticed a "new hum" developed at a certain RPM. Because this a boat, engine noise is actually easier to hear and feel. The damper is also closer to your ear, right there behind you when driving. Nate commented that the hum was the damper doing it's job. But Dave had never noticed this hum before.

To me that statement could be a key to this specific failure. The hum wasn't present before, next weekend that crank breaks within a couple minutes of firing up. Was it actually a harmonics issue when the hum developed and it failed later, or was that hum a sign it was already broken?

From everything I've heard and read harmonics is likely a key to this fatigue, and the AF cam does change those somewhat. So maybe we should track the damper models ran on each failure, along with firing order and crank balance.

I tend look at other models like my inline 6 CDI Mercedes which you could stack wine glasses on the hood at 4500R's. Removing vibration harmonics from this 90deg V8 shouldn't be that hard since it has a "evenly spaced firing order" to start with. Besides that, I feel the rest is likely journal size as Sting has been researching.

Mark Broviak and Fingers have looked at how just about every built engine has a different resonant harmonic due to differences in firing order, block fill, balancing, injectors, tuning, dampening, etc. I'm sure Jeremy Wagler has too, along with some other guys. But that got me questioning the effectiveness of the ATI because if they are tuned for X and Y frequency specifically, because they were chosen values after seeing the highest amplitudes during testing at the factory, and those thresholds shift after making changes A, B, and C to the engine/tuning, then the ATI is rendered ineffective. Whereas a Viscous dampener can be designed to calm a wider range of frequencies and should be spec'd to each engine individually in my opinion.

 

[Burn Down]

I stumbled across this the other day. While I realize it's a Corvair engine in an airplane, I couldn't help but see the striking resemblance in crank fractures... Also has some merit to what Fingers was discussing on a support bearing for the front of the crank.
http://www.n56ml.com/corvair/crankshaft/

 

[Chevy1925]

I stumbled across this the other day. While I realize it's a Corvair engine in an airplane, I couldn't help but see the striking resemblance in crank fractures... Also has some merit to what Fingers was discussing on a support bearing for the front of the crank.
http://www.n56ml.com/corvair/crankshaft/

now that is a cool read!

ive often wondered why no one has tried a wider bearing on the front main.

what are the thoughts on re-notching the grooves for running two bearings up there, drilling the edge for the oil passage and cutting the edge down to make the bearings fit inside cap? the surface area is there already... id honestly pull this engine i have on the stand apart to try this if its a feasable idea. i get some break else where on the crank but if we have the area to work in, i dont see why it wouldnt help elimiate a failure point and get more run time from a crank

 

[Fingers]

IF you read again, you will see that the extra bearing was NOT the answer. The Prop loading was contributing, but not root cause.

 

[Chevy1925]

IF you read again, you will see that the extra bearing was NOT the answer. The Prop loading was contributing, but not root cause.

I never said it was Jon. It's one part to a few fixes he did. Just as the af cam is not the all out fix.

I'm asking why it would t help add more time with the combo of other parts we have done. If you can add the addition bearing surface easily on a build, what is the down fall to it? GM made that main pretty damn wide...... Why?

 

[Yellow Jacket]

I never said it was Jon. It's one part to a few fixes he did. Just as the af cam is not the all out fix.

I'm asking why it would t help add more time with the combo of other parts we have done. If you can add the addition bearing surface easily on a build, what is the down fall to it? GM made that main pretty damn wide...... Why?

From from 6.2 experiences lol. They would break webbing in block.... AM General's fix after purchasing for military was to simply tap the mains for next smaller bolts... The added meat in the mains (from the smaller diameter hole) and no more cracking above the main webbing...

 

[Chevy1925]

From from 6.2 experiences lol. They would break webbing in block.... AM General's fix after purchasing for military was to simply tap the mains for next smaller bolts... The added meat in the mains (from the smaller diameter hole) and no more cracking above the main webbing...

the front main is almost twice the size of the rest. you think engineers really though cracking would be an issue there? thats cost they could have saved on since cracking isnt an issue on the smaller mains.

Guy and i talked about this a few years ago and didnt seem there was a definitive answer on it

i see it more as "the space is there, would taking advantage of it offer no help?"

 

[kidturbo]

Don't know how far anyone actually dove into measuring RPM based harmonics and crankshaft torsional vibrations on the AF cam research, but I had a nice chat yesterday with someone who has the right tools to measure such values. They can measure deflection and rebound along a crankshaft as each cylinder fires in real time. Which can then be plotted and compared to measured engine harmonics.

Another part of our conversation revolved around how torsional twist and vibrations might be effected by different tuning aspects. Since changes to timing ultimately effects cylinder pressures at specific degrees BTDC. These tuning variables along with the stock crank design, balancing options, and accessory driven loads relating to crank fatigue is what we spent the better part of an hour discussing.

What we agreed that might be a good approach to understanding these crank failures better is to take a couple engines and put them on a harmonics tracking dyno setup, add fingers cylinder pressure mapping capabilities, and then log different tune configurations. Once several runs are mapped this way, swap in a AF cam and run it all again. Then switch to a internally balanced engine and repeat.

Might sound like big project, but nothing a few good wrenches and a couple computer geeks couldn't get together and handle over a weekend. Anyone interested if the price is right?

 

[RichLMM]

can add me to the list

 

[blue09dmaxx]

can add me to the list

In what

 

[RichLMM]

In what

my puller 01 Lb7 motor

 

[Yellow Jacket]

my puller 01 Lb7 motor

Well that's because you had a used crank....just so you know