Broken SBC crankshaft

[Mike Pearson]

Originally Posted by Mike Taylor 3601

if crank wasn't magnafluxed when ground, may have been cracked and then broke.
#4 main bad and others good sounds like crank was cracked and rubbing bearing before it broke, if look closely may see where #3 and # 5 main bearings have been rubbed some,most of the time when you have a few main bearings rubbed/wiped out the crank will be cracked and flexing.
Was the radius left in crank when reground?If had 1/8 radius and it was cut less or square that weakens the crank also.
When I tear into engine with broken oil pump,I check balance of rotating assembly,clearance between oil pump shaft and block and oil pump pick up/pan clearance.
but there is chance since most bearings were OK that crank broke and vibration from that broke oil pump.
Mike Taylor 3601,

Unfortunately we did not magnaflux the crank before it was ground. The radius on the crank was real nice when we got it back from the grinder. The break was right in the same area that would have been damaged from the oil pump breaking off. the burned bearing has me puzzled though. Maybe the cracked crank was pushing some metal through the system but the two rod bearings next to the broken area were still good. The newer model of the Melling M55 pumps are known to break. I replaced that with a moroso billet pump on that last freshen up. I also put a low oil pressure warning light in the car. I shut the engine down as soon as that lit up.