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Old 10-15-2010, 09:53 AM
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Cracked crank questions

I have a forged crank that has a crack on one of the journals. It's not a horrible crack, but it's cracked nonetheless. I've heard some people say the crank is junk and use it as a (insert object here). Others have said that if it's a street motor that won't be seeing above 5500-6000, it should be fine.

This crank is already .030./.030, so if it's not usable, it's not a great loss, but for this particular engine, cranks aren't growing on trees. So if it's usable, that would be great.

I was planning on building an engine for street driving, so the maximum RPM it would see would be right at 5500-6000, and was just curious as to what people thought about using a cracked crank. I know the first thought is "Throw it out!", but I want to hear from people that have run a cracked crank or worked at a shop that dealt with them and know why you shouldn't run one or something like that.

Thanks!

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Old 10-15-2010, 10:28 AM
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Nothing highly personal or direct . . . But about 40 years ago, a friend was just cruising down I 280 in his TR4, on the ramp headed for I 880. Loud noise. Vibration. Drove home very slowly. We tore the engine down . . .Broken crank, in a stock engine. I'm an engineer . . I work with large forgings and castings on a near daily basis, in a defense industry. I am not a materials engineer, or metallurgist, but I can very confidently say that my friends broken crankshaft started as a crack. I personnally think it's amazing that he made it home on engine.

Maybe Scat makes something that'll fit?

Pat
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Old 10-15-2010, 10:45 AM
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Before you use it or throw it away I would ask around and search for the best crank shop in your area. Take it to them, have it mag'd and let THEM tell you if it's repairable. If the crack is not too deep they may be able to weld and turn that journal....but remember, if it's cracked in one place in may have others that you can't see. Ol' Mr. Magnaflux will let you know.
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Old 10-15-2010, 11:24 AM
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Quote:
Originally Posted by imacarfan2
I have a forged crank that has a crack on one of the journals. It's not a horrible crack, but it's cracked nonetheless. I've heard some people say the crank is junk and use it as a (insert object here). Others have said that if it's a street motor that won't be seeing above 5500-6000, it should be fine.

This crank is already .030./.030, so if it's not usable, it's not a great loss, but for this particular engine, cranks aren't growing on trees. So if it's usable, that would be great.

I was planning on building an engine for street driving, so the maximum RPM it would see would be right at 5500-6000, and was just curious as to what people thought about using a cracked crank. I know the first thought is "Throw it out!", but I want to hear from people that have run a cracked crank or worked at a shop that dealt with them and know why you shouldn't run one or something like that.

Thanks!
Unless this is some sort of an unusual and rare crank, you're money ahead to just replace it. The cost of divining how extensive the crack is, grinding it out if it is repairable, welding up the journal, then turning and polishing it to a restored condition will most likely exceed the cost of replacement.

Always keep in mind that a major bottom end failure of rods or crank is almost always fatal to the block, usually gets the cam, and frequently takes out the heads as well. Consider the cost of this when thinking about the money saved cutting corners on rods, crankshafts, pistons, pins, and bearing caps.

Bogie
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Old 10-15-2010, 03:37 PM
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Cracks radiating from an oil hole will not usually hurt in a street engine. I have salvaged plenty of expensive stroker cranks suffering from this malady. If the crack is at the outer edge of the journal or into the fillet. Throw it away!!!
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