Crank failure

General engine tech -- Drag Racing to Circle Track

Moderator: Team

Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Crank failure

Post by Steve.k »

Any specific reason why crank would break in front of front main? 700hp na? Forged Steel.? 438 inch sbf.
mag2555
Guru
Guru
Posts: 4584
Joined: Sun Sep 18, 2011 11:31 am
Location: Heading for a bang up with Andromeda as we all are.

Re: Crank failure

Post by mag2555 »

The stress load from a non radius for the transistion from counter weight cheek to Crank snout ?
Can you post up a shot?
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
ClassAct
Guru
Guru
Posts: 1024
Joined: Wed Dec 05, 2018 11:55 pm
Location:

Re: Crank failure

Post by ClassAct »

Damper wasn't up to the job????
Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Re: Crank failure

Post by Steve.k »

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! #-o
Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Re: Crank failure

Post by Steve.k »

ClassAct wrote: Mon Aug 19, 2019 12:06 pm Damper wasn't up to the job????
Year old Romac usually a good piece!
lance flake
Pro
Pro
Posts: 286
Joined: Tue Apr 01, 2008 12:14 am
Location: union ms

Re: Crank failure

Post by lance flake »

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
PackardV8
Guru
Guru
Posts: 7619
Joined: Sun Jul 30, 2006 2:03 pm
Location: Spokane, WA

Re: Crank failure

Post by PackardV8 »

Steve.k wrote: Mon Aug 19, 2019 11:55 am 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.
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Re: Crank failure

Post by Steve.k »

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.
Racer71
Pro
Pro
Posts: 324
Joined: Mon Jun 24, 2019 12:36 pm
Location:

Re: Crank failure

Post by Racer71 »

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.
lance flake
Pro
Pro
Posts: 286
Joined: Tue Apr 01, 2008 12:14 am
Location: union ms

Re: Crank failure

Post by lance flake »

How much run time?
Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Re: Crank failure

Post by Steve.k »

Right around 300-400 passes between 1/8 & 1/4. Majority on 1/8. Likely only 20 on 700hp tune 650 before that.
1972ho
Guru
Guru
Posts: 1299
Joined: Thu Nov 21, 2013 12:52 am
Location:

Re: Crank failure

Post by 1972ho »

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.
You do not have the required permissions to view the files attached to this post.
PRH
Guru
Guru
Posts: 1501
Joined: Fri Sep 08, 2017 4:16 pm
Location: S. Burlington, Vt.

Re: Crank failure

Post by PRH »

I have seen some timing gears rub the radius on the crank post and leave strees rizers
I’ve seen this myself on a BBC with a USA made 4340 crank that had the snout broken off.

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.
Steve.k
Guru
Guru
Posts: 1562
Joined: Sat May 28, 2016 10:41 am
Location:

Re: Crank failure

Post by Steve.k »

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! :cry:
User avatar
Rick!
Expert
Expert
Posts: 529
Joined: Fri Jan 01, 2016 4:13 pm
Location:
Contact:

Re: Crank failure

Post by Rick! »

PackardV8 wrote: Mon Aug 19, 2019 12:26 pm
Steve.k wrote: Mon Aug 19, 2019 11:55 am 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.
Post Reply