Crank failure
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Crank failure
Any specific reason why crank would break in front of front main? 700hp na? Forged Steel.? 438 inch sbf.
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Re: Crank failure
The stress load from a non radius for the transistion from counter weight cheek to Crank snout ?
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Re: Crank failure
Not apart yet just had fail over weekend broke off and harmonic balancer tilted up against water pump suction pipe and killed engine. Wiggles around good but assuming gear and chain holding from flying completely out. Pics later today mag2555!
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Re: Crank failure
Seems to be common place for sbf cranks to break. I have seen some break the post off also. I have seen some timing gears rub the radius on the crank post and leave strees rizers
Re: Crank failure
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.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Studebaker-Packard V8 Limited
Obsolete Engineering
Re: Crank failure
This was a aftermarket forged steel 4.25 stroke. Yes from across the ocean. But appeared to be be decent looking piece. I wont mention any names till i get it out and see if any tell tale signs or something i maybe did wrong. Been in a fair while. But have been adding power to it. Maybe found plateau of its design.
Re: Crank failure
I bent one badly in a 396 Windsor. It was still 50 oz ext balance was the only reason I figured it bent. That was a cheaper cast scat crank though.
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Re: Crank failure
Right around 300-400 passes between 1/8 & 1/4. Majority on 1/8. Likely only 20 on 700hp tune 650 before that.
Re: Crank failure
I had questions about why my V-6 crankshaft broke after only about 10 -12 passes and thought that it had something to do with the fluid damper and the way the crank was balanced,per what scats advice was to not to drill more than two /three holes in the counterweight can cause a crank to break.
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Re: Crank failure
I’ve seen this myself on a BBC with a USA made 4340 crank that had the snout broken off.I have seen some timing gears rub the radius on the crank post and leave strees rizers
Checking this gear chamfer/crank radius relationship on subsequent builds of various brands of motors showed this was not an unusual situation, and really should always be checked.
I usually end up chucking the gear in the lathe and using a sandpaper roll in a die grinder, rounding off the edges of the chamfer.
I’ve seem them where there is enough interference between the chamfer on the gear and the crank radius that you can slide a feeler gauge in behind the gear when it’s on as far as it will go.
After a little radiusing of the chamfer on the gear, the gear will sit flush against the crank.
Somewhat handy with a die grinder.
Re: Crank failure
Ok thanks everyone. Ill get some pics later today. First one I broke. Glad it was my own and not one built for someone else!
Re: Crank failure
1.75²/1.375² DNE 28%. Straight up diameter to diameter, yes, but area is a different calc.PackardV8 wrote: ↑Mon Aug 19, 2019 12:26 pmMaybe, 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.