cracks in crankshafts


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By NoShortcuts - 13 Years Ago
It seems like several Forum regulars have written in the last 6 months about their experiences with crankshafts actually breaking while at a machine shop or being found to be cracked before or after remachining. Mike (46yblock) and Oldmics have been the most recent, and there was a third posting not too long ago from someone in (?) New Hampshire, I think.



Someone wrote recently about the value of taking a CLOSE look at cranks that have had the radiator cooling fan mounted on their front snout as apparently was the case on some large truck applications.



How wide spread IS this fatigue problem? Is it more common with 312 or 272/292 cranks? Is there a common breaking point, like a particular crank throw?



Is there value to having a used, 50+ year old crank 'destressed' by shot peening or a heat treat process like normalizing?



Input appreciated. -I HATE Angry doing things twice and don't like thinking about the potential collateral damage and cost of having a crank break in the fresh build I'm embarking upon.



Regards,
By Hoosier Hurricane - 13 Years Ago
No Shortcuts:

My experience with cracks in 312 cranks from my race car engines has been with cranks that have been reground.  It seems most regrinders do not dress a radius on their wheels, and cracks show up after running them in the race car.  The cracks begin as little hairline cracks at the journal and main where the radius should have been, and in time they "connect the dots" and break.  The last couple of cranks I have had ground I insisted on a generous radius, and have seen no cracks.  A heavy duty truck crank would probably be subjected to as much stress as my race cranks over their lifetime, radius or no.

By 46yblock - 13 Years Ago
I dont have anything of value to add, except...most everyone seem to agree that a crank accidentally knocked over from a standing position isnt expected to break in two, especially after it has been maged.  But like my machinist said, strange things can happen.  He relayed a story about a customer who worked on a very high something or other.  At lunch they would get jolleys by dropping the coke bottles to the concrete below.  Once, the bottle didnt break Hehe .

2-3 years ago I was in a very junky shop, with stuff everywhere and difficult to walk in.  I accidentally brushed against a standing crank and it clanged to the floor.  "Dont worry" said the owner.  Now I do worry a little.

Like John Mummert told me once, everything breaks eventually.  New cranks would be nice.

The HD 292s with snout mounted rad fans were forged, so they were extra stout.  The cast 312 crank of mine that broke had been intalled in a COE, in place of the HD 292.  So it wasnt meant for the application, though no doubt it gave the drivers a thrill.  Then it went into an F350, still retaining the heavy fan and hub on the crank.  I am happy it broke into two.