BBC broken crankshaft

MEXJOE · 08-29-2013, 10:30 AM

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 · 08-29-2013, 11:08 AM

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

MEXJOE · 08-29-2013, 11:41 AM

Quote:

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 · 08-29-2013, 02:50 PM

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 · 08-29-2013, 04:14 PM

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 · 08-29-2013, 09:15 PM

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?

Quote:

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.

08-29-2013, 11:08 AM	#2
njk53 Senior Member Join Date: Jul 2007 Location: East China, MI Posts: 504 Likes: 0 Liked 0 Times in 0 Posts	Re: BBC broken crankshaft Steel or Cast Iron. How old is the crank and what is the max engine RPM? __________________ Nelson Kowal Stock 345

08-29-2013, 02:50 PM	#4
njk53 Senior Member Join Date: Jul 2007 Location: East China, MI Posts: 504 Likes: 0 Liked 0 Times in 0 Posts	Re: BBC broken crankshaft 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. __________________ Nelson Kowal Stock 345

08-29-2013, 04:14 PM	#5
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,110 Likes: 1,570 Liked 1,808 Times in 411 Posts	Re: BBC broken crankshaft 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. __________________ Alan Roehrich 212A G/S

08-29-2013, 10:30 AM	#1
MEXJOE Senior Member Join Date: Jun 2008 Location: nhranumbers.com Posts: 875 Likes: 137 Liked 165 Times in 51 Posts	BBC broken crankshaft 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.