BBC broken crankshaft

[MEXJOE]

Originally Posted by njk53

Steel or Cast Iron. How old is the crank and what is the max engine RPM?

STEEL, 7000 RPM, used OEM CRANK (as per rules at the time), IN ENGINE racing for about 300 passes.

[njk53]

The steel cranks that were used for production back in the day were satisfactory but, you have to remember, they were designed for heavy duty passenger car usage. The forgings could have contained small stress cracks that would never amount to anything under ordinary passenger car usage. Now you are stressing the crank even worse now with a violent twisting action on launch and spinning the crank 7,000 rpm producing harmonics the crank probably has never seen in it's lifetime. The fact is these small stress cracks could have developed into big cracks through fatigue over the years and eventually resulted in the catistrophic failure you experienced.

Another thing, back in the day, crankshaft forgings were not an exact science and the forging process was crude at best. Metallurgy has also significantly changed over the years. The steel recipes have improved dramatically.

I was just recently reading that Eagle now manufactures a cast crank that is as good or better than a factory steel crank.

[Alan Roehrich]

If it pulled a rod journal/throw out of the crank, suspect a piston/wrist pin problem.

Most often, a crank breaks at the front of #1 journal or the rear of #8 journal if the crank itself is the root cause of the failure. If it breaks at #1, the balancer could be the problem, at #8 the problem is load or fatigue.

About 9 out of 10 times I see a rod throw broken out of a crank, I see a wrist pin or maybe piston problem.

[MEXJOE]

It did rip one wrist pin out of the piston. What happened first I don't know.
I just thought it was from the crank exiting with the 5/6 rods in such a quick fashion?

Originally Posted by Alan Roehrich

If it pulled a rod journal/throw out of the crank, suspect a piston/wrist pin problem.

Most often, a crank breaks at the front of #1 journal or the rear of #8 journal if the crank itself is the root cause of the failure. If it breaks at #1, the balancer could be the problem, at #8 the problem is load or fatigue.

About 9 out of 10 times I see a rod throw broken out of a crank, I see a wrist pin or maybe piston problem.

[Adger Smith]

I had a Boat racing customer do that to a 454 steel factory crank, but it was because of a NOS malfunction at about 6,000 rpm.
How much NOS did it take to get yours to come apart? ;~)

[Jim Cimarolli]

Glen,

Sorry to hear about the carnage. Wish I could say that it never happened to me.
From my experience, those GM steel cranks are a twisted forging, so I have had better luck with the cast cranks.

[Mike Taylor 3601]

I don't know if you ever wet magged your crank,if you did'nt you should everytime it is out,every crank that comes through my shop gets magged even if comes in for a polish,
I've used alot of GM steels over the years,the black cranks(tufftrided) always seemed a little better,once you got them straight,most of them had around .003-006 runout on mains when new,always was told from tufftride process?
Was your engine external balance? External balance 454 and 400 small blocks always had more crank trouble than internal balance 396/402/427.
I've always been told that ext.balance flywheel/balancer exerts force on crank because they are trying to go horizontal,and puts more pressure on crank,we always called it gyroscoping effect,I don't know if that is scientific term,or Hillbilly slang

So does Eagle have translators or speak chinese so they can know about the processes their cranks go through during the casting/forging process LOL

MikeTaylor 3601