Like many things on the internet, I ran across this post while looking for something else. I realize it is nearly 10 years old, but hey... better late than never.
I experienced a similar failure several years ago on my 44 GMC, with a 270. The engine had been rebuilt by a PO sometime in the past. Unknown mileage - although based on internal wear, I don't think it had a ton of miles on it since rebuild.
For a week or so prior to the failure, I thought that I heard a very, very faint and intermittent clunking noise under acceleration. Then, one day under normal driving, there was a large sickening bang, followed by a severe knocking noise.
Dropping the oil pan revealed that the crankshaft had broken just aft of the #3 main bearing journal. The fracture extended through the counterweight up to the #5 rod bearing. Unsupported, the crankshaft section with the #5 and #6 rods wobbled and cracked the rear main bearing cap in several places. It also cleaved out a piece of the camshaft, which punched a hole in the block.
Inspection of the crank revealed that it had been ground 0.020" undersize, and whoever ground it made sure that the journal to throw intersections were nice, square 90 deg corners - no radii at all. In looking closely at the failure, it appears that the crack started at the #5 rod journal, then propagated across the oil passage and down to the #3 main bearing. My theory is that this was caused by the failure of the crankshaft grinder to maintain a radius along the journal corners, which caused a stress concentration at the rod journal, resulting in a crack which grew until the point where the crank lost enough strength that it finally failed.
WRT any surface hardening, I had always wondered about any possible ramifications related to loss of surface hardness when grinding these crankshafts. So, I talked to the metallurgy professor at the community college where I teach part time. This guy actually worked as a metallurgist for GMC on cranks and other engine parts in the 70s and 80s, so I trust his judgement.
Always on the lookout for interesting projects for his students, he offered to let them section the crank, and characterize the area of the fracture, along with some of the other journals. Their findings were that it was just a steel forging, with no appreciable post manufacturing surface heat treating.
I asked if perhaps any surface treatment could have been removed when the crank was ground, but he did not seem to think so. His opinion was that the heat treatment techniques of that era, if they had been applied, would have likely extended deeper than 0.020". He also felt that it was unlikely that any "topical" surface treatments such as nitriding, would have been applied to a crank of that era - assuming it was a stock crank, which I believe it was (It had a PN on it)
On a side note, I also let his students section the broken piece of camshaft, as I had also wondered about the effects of cam grinding. His response was extremely surprising - even to him. Apparently, the camshaft in that engine was cast iron with absolutely no surface heat treatment whatsoever. He kept asking me the mileage on the engine (Which I did not know) as he felt that it was unlikely this cam would have gone even 10K miles, before it experienced severe wear on the lobes. This cam had "USA" cast into it, but was not marked "GMC" nor did it have a PN cast into it like on other OEM camshafts that I have seen. So I guess the message is buyer beware, when purchasing old pats for these engines.
Best Regards…
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