Broken SBC crankshaft

[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,

[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.

[BillK]

Originally Posted by Mike Pearson

Unfortunately we did not magnaflux the crank before it was ground.

That's why I like the guy that grinds cranks for my shop. He simply WILL NOT grind a crank without magnafluxing it first. He has found plenty of cracked ones over the years, they go in the scrap pile

[Alan Roehrich]

It has been my experience that the vast majority of crankshafts that fail from fatigue fail either in front of #1 rod journal, or behind #8.


For an aftermarket crank, I strongly suggest Callies, I don't think you'll find anything better for Stock Eliminator or Super Stock.

[nhramnl]

You probably already know this, but just in case... Be sure to have the converter, front pump bushing and front pump checked (assuming the car uses an automatic trans) before you put the car back together. With the rearmost chunk of the crank wobbling around when the failure occurred, they may have gotten hurt.

[dmir55]

A quick way to check for a crack on a forged crank is to hold it off the ground and tap it with a hammer. If it rings like a bell it's good. I it gives a thud sound it's cracked. If it's a cast crank you have mag them.

[JimL]

Never reuse a crank, especially in a racing motor, without magging it even if you are not grinding it or even if it did not blow up.