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Old 10-02-2016, 05:30 PM   #1
35cab
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Default Catastrophic crank failure.

I thought I would share a couple of photos. French block 4" crank in my 35. Whilst on a trip in August I felt a vibration, couldn't work it out, when car got to around 65 vibration went away. 200 miles into my 350 mile drive home to Scotland, had to slow down for roadworks then all hell broke loose.
Undignified return on back of recovery truck.
Pulled the engine last week, could see no obvious problem, checking for crank play float noticed a discrepancy between measurement on the front and at the rear, undid rod cap, ouch! (pic 1) crank out and this is what I found (pic 2)
JB weld ? :-)
I need to work out how this happened.
Gordon.
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Old 10-02-2016, 05:38 PM   #2
40cpe
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Default Re: Catastrophic crank failure.

I'm sorry to see that. Please keep us informed as you find the root cause. I can think of a couple of possibilities, I'm sure the more experienced builders will have some suggestions.
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Old 10-02-2016, 05:40 PM   #3
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Default Re: Catastrophic crank failure.

You're very fortunate that the thing didn't let go and destroy the block (at least you didn't mention that!).

Before I pass any judgement - whose crank is it? Is this an original Merc, or a SCAT or Eagle? Also, when the engine was built, was the lower end balanced?

I will say - that is a BAD day to take an engine apart and see that stuff! BUT, it could have been a LOT worse - you could have had rods sticking out of your oilpan!
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Old 10-02-2016, 06:16 PM   #4
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Default Re: Catastrophic crank failure.

Is this an engine you rebuilt? Was the crank turned for new bearings? If so, did the crank grinder put the appropriate champher on the throws to prevent a stress point? Man, you are amazingly lucky it lasted that long. The rod held it all together.
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Old 10-02-2016, 06:24 PM   #5
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Default Re: Catastrophic crank failure.

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Originally Posted by 35cab View Post
JB weld ? :-) Gordon.
That can be fixed.
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Old 10-02-2016, 10:42 PM   #6
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Default Re: Catastrophic crank failure.

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Damn cheap foreign parts!
Was this a French crank as well?
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Old 10-02-2016, 10:48 PM   #7
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Default Re: Catastrophic crank failure.

Brother, I feel your pain!! That is unusual. I'm interested in whether that was a French crank too; my French crank didn't look like that; it was cast steel and the outermost portions of the counterweights had been turned.
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Old 10-02-2016, 11:00 PM   #8
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Default Re: Catastrophic crank failure.

look at that sharp corner on the journal. Your rod bearings probably have an 1/8" radius on them and the crank should have the largest radius possible, like 3/32" r. That sharp corner lends itself to fracturing. Make sure your new crank has nice big radii in the corners but not too big to bind the bearings.
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Old 10-03-2016, 02:05 AM   #9
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Default Re: Catastrophic crank failure.

Can you post pictures of the break? Was there an old crack?
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Old 10-03-2016, 03:12 AM   #10
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Default Re: Catastrophic crank failure.

Here's a pic of the break

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Old 10-03-2016, 04:38 AM   #11
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Default Re: Catastrophic crank failure.

You might want to back off on the nitro percentage.
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Old 10-03-2016, 04:40 AM   #12
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Default Re: Catastrophic crank failure.

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You might want to back off on the nitro percentage.
Or less boost!
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Old 10-03-2016, 05:16 AM   #13
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Default Re: Catastrophic crank failure.

as far as I know crank is a std French 4" crank (its not an aftermarket crank) which had been ground when the engine was built, the engine was balanced when it was built. the car has been running for 4 years now? maybe longer and never had any issues previously. I'm sure the engine is pretty standard apart from an isky cam and a pair of twin carbs on a thickstun PM7 manifold.
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Old 10-03-2016, 05:23 AM   #14
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Default Re: Catastrophic crank failure.

Henry was a stubborn sort. One of his points was the three point mounting of suspension and engines. Another was hatred for six cylinder engines. Unfortunately, another was his insistence on three main bearing engines.
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Old 10-03-2016, 06:46 AM   #15
Kurt in NJ
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Default Re: Catastrophic crank failure.

From what I see there is a sharp corner where there should be a radius----i was checking my collection of cranks and the only ones cracked had been reground without a radius-----I have been teaching myself to run a crankshaft grinder and it takes a lot of dressing to change from a "chevy" radius to the early Ford radius---and grinding wheels are $$----I can see the burn marks on the thrust flanges too from the grinding wheel being hard against them
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Old 10-03-2016, 07:25 AM   #16
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Default Re: Catastrophic crank failure.

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I can see the burn marks on the thrust flanges too from the grinding wheel being hard against them
I don't believe the marks you can see are from the grinding wheel, none of the other flanges have any marks on them. I think those marks are from the rod(s) when the crank has broken.
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Old 10-03-2016, 08:40 AM   #17
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Default Re: Catastrophic crank failure.

It looks like the crack started in the radius at the bottom of the throw then progressed a good bit until the remaining bit twisted off. The radius on the good side doesn't look very big but photos can be deceiving. It's really hard to determine exactly when the crack started. A person has to do a good Magna-flux of the crank before spending the bucks on machining. If it was cracked before machining, it should have been caught then. I can't see any inclusions in the metal but I'm not looking directly at it either. Magnified inspection can usually tell exactly where the crack started.
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Old 10-03-2016, 09:10 AM   #18
aonemarine
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Default Re: Catastrophic crank failure.

simplest answer.....its 70 frigging years old!
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Old 10-03-2016, 10:06 AM   #19
Bored&Stroked
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Default Re: Catastrophic crank failure.

I agree on the comments about the fillet/radius in the journal corners - there isn't hardly any to be seen. This creates a heck of a weak spot.

Just for comparison, here is the 4" Merc crank that I just put in my 284 cube 42 Merc engine last year - the crank has been offset ground/stroked to 4 1/8".

Notice the nice radius in the rod journal corners - something that I always have done (have a REALLY good crank grinder here in Ohio).

10-3-2016 11-02-51 AM.jpg
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Old 10-03-2016, 10:20 AM   #20
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Default Re: Catastrophic crank failure.

Are you aware of the engine ever experiencing a sudden stoppage and or the crank ever having been dropped/ fallen over?
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