BBC broken crankshaft

[MEXJOE]

How many here have seen a BBC broken crankshaft in a stocker?
Mine snapped the 5/6 rod journal off and whipped it straight down to the pan.

[njk53]

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

[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

[Alan Roehrich]

The old 6223 forging, which many of us have used for years (we have a couple with several hundred passes on them), are a 1053 alloy twisted forging. These are the cranks originally used in most 396 and 427 high performance engines and the medium truck engine of days past. They may or may not be "TuffTrided" which was a GM trade name for nitriding. The "TuffTrided" cranks are a darker gray in color, and are a little smoother, because they may also have been shot peened and inspected, similar to the "Pink" rod and the "dimple" rod.

There is another older forging, 7415 is the correct number, I believe, that was used in the L-88 and ZL-1 engines, it is shot peened, "TuffTrided", and cross drilled.

The newer 6223 forging is 5140 steel, and it is a non twist forging. It will have a very different appearance from the earlier cranks, the forging number is "pressed in" rather than raised as it was on the old forgings. The new forgings were made for the newer medium truck engines. They are 5-7 pounds heavier than the older forgings.

Eagle makes a fairly nice 4340 forged crank for the 396 and 427. However, it is heavy, none of the rod pins are drilled, and they profile the counterweights the same as a 454 crank. The "flats" on the counterweights are cut 1/8" too close to the main journal centerline, as they would be to clear the bottom of the piston on a 4" stroke 454. This forces them to make the counterweights longer in order to offset the weight of the rod throw. That is made worse by the fact that even though GM did drill the front and rear rod pins, Eagle does not drill any of them, even though the rest of their forged cranks have all four rod pins drilled. I got one of their very first NHRA approved cranks. It works okay, but I was very plain spoken and critical when I called them and told them what I thought of it. They were polite and said those were valid criticisms. I doubt they'll do anything about it.

Callies should have their crankshafts for Stock Eliminator ready soon, if they are not already.

Remember, just because a crank is approved for Super Stock does not mean it will pass tech in Stock Eliminator. Some of the "off the shelf" crankshafts that are approved for Super Stock have "options" that are NOT legal in Stock Eliminator. We have a Bullett prepped GM 6223 forging that is a super nice piece, and was bought for Super Stock, it is NOT acceptable for Stock Eliminator according to 2-3 tech inspectors I asked about it, because we were just going to use it instead of buying an Eagle, and buy a super nice peice for the Super Stock engine.