Crank failure

[PackardV8]

Any specific reason why crank would break in front of front main? 700hp na? Forged Steel.? 438 inch sbf.

Maybe, it's just exceeding the limits of the SBF crank snout and damper architecture, designed for sub-300", sub-300hp. For comparison of what engineers provide when they know that area will be heavily loaded for long duration, look at the crank snout on an FT 361"-391" truck engines. It's 1.75" diameter, vs 1.375" on a stock SBF, a 28% greater area cross-section and diameter.

1.75²/1.375² DNE 28%. Straight up diameter to diameter, yes, but area is a different calc.

For true; don't trust math by someone who went to high school more than seventy years ago, but your diameter squared formula won't get us there either.

The formula for area of a circle is A=πr2:
.875 x .875 x 3.14 = 2.4
.6875 x .6875 x 3.14 = 1.48
2.4 - 1.48 = 1.2
1.2 / 1.48 = 62% greater area

[jake197000]

maybe it was just a faulty crank.it happens.

[superpursuit]

I assume that you were NOT running it externally balanced at that power level?? No crank likes that.

You mentioned that it is an offshore crank that you won't say any names.

If it starts with pro then this is normal from what I've seen.

[ssmmith]

As has been mentioned, make sure the bottom gear radius matches or exceeds the radius on the nose of the crank. some crank companies are running an extremely long harmonic balancer bolt that actually goes into the front main of the crankshaft and sometimes into the front counterweight, at first I thought it was overkill, but it pulls the nose towards the crank making a stronger method of attachment. Also if externally balanced will make a difference. Good luck and I hope you didn"t kill too many other parts. Steve

[BillK]

Steve,
I have seen one before that a customer brought in. He had used a 9 keyway crank sprocket and you could see the indentation of all of the cutouts all the way around the crank and that was exactly where it broke. Same deal as someone else mentioned, not enough chamfer on the sprocket to allow it to sit flat.

[leahymtsps]

Steve,
I have seen one before that a customer brought in. He had used a 9 keyway crank sprocket and you could see the indentation of all of the cutouts all the way around the crank and that was exactly where it broke. Same deal as someone else mentioned, not enough chamfer on the sprocket to allow it to sit flat.

X2 I've seen the same with a 9 tooth gear with the same results

Tom

[Steve.k]

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wasn't what i thought guys. It was a rpm crank maybe found limits?

[Steve.k]

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. I should have known something up. It slowed a bit but couldn't figure why. Must've cracked putting pressure on front main. Then once main spun and started moving she let go more. Timing chain let go and that was that.

[Steve.k]

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each bank not sure if it cleaned soot off before or after chain let loose. Other than rich she was tight.

[1972ho]

That’s about where my V-6 even fire crank broke was this a standard cleveland journal size or smaller mine was a 2.100 journal that was .905 wide.

[ProPower engines]

Steve,
I have seen one before that a customer brought in. He had used a 9 keyway crank sprocket and you could see the indentation of all of the cutouts all the way around the crank and that was exactly where it broke. Same deal as someone else mentioned, not enough chamfer on the sprocket to allow it to sit flat.

Same thing I have seen as well. But also fluid dampers will snap the nd of a crank like that on decel as well.
Too much weight on the end works against the crank when it tries to unwind...............snap-0

[Steve.k]

Was a good name cleveland balancer and everything neutral balance so who knows. It made 703 at sea level but was way below that at this altitude.

[ssmmith]

Steve, I have been around crankshafts for a little while. The one that I broke like that I Know for a fact that it was balancer related. It broke right in the same place as that one. This was one that I made from a raw forging. It was a lot thicker on the front rod cheek than that one. the outer ring of my balancer came off and I ran it anyway. I was young and thought it wouldn't hurt for a week or two. Since that I have only ran ATI balancers and haven't broke one since in that place. I did break a stock cast 454, but it was making a little more power than a cast crank could hold for about 3 or 4 years. It broke at the rear rod. I know ATI is an expensive piece but worth it.
Good luck, Steve

[Steve.k]

Thanks ssmmith it could be. I had a different balancer on this motor that spun. Maybe it started the problem?Hopefully i can fix block. Took a little material out of block main saddle on chain side.

[Kevin Johnson]

Any specific reason why crank would break in front of front main? 700hp na? Forged Steel.? 438 inch sbf.

IF the break is aligned with the terminus of the snout bolt THEN the stress raiser caused by the deformation of the initial fastener threads could finally have done it. Hypothesis.