Catastrophic crank failure.

[DocJohn]

From what I can see from the pictures the cracks have propagated under an oscillating torsional loading, rather than a bending load. Yes, they almost cetainly started at the sharp radius but probably would not have done so had there been a crank damper.

[Ol' Ron]

During the early 60's I started racing at the Danbury speedway in CT. It was a sportsman division and some modifications were allowed. Bore to 3 5/16, stock 33/4 stroke, any cam, stock size valves, stock heads and intake,Any carb that bolted to the stock intake. These rules lasted until the end of the flathead ears, with only one change, Hi Compression heads, because there were so many cheater sets around.

In the beginning we just stretchered the relief spring on the pump out a mile, or shimmed it. Some guys ran as much a 100 lbs of oil pressure. Every once in awhile a crank would brake, or a few rods would vacate the pan. Well it finally happened to us. at that time I was working for the Bullard machine too Co. and took the broken crank into the foundry inspection shop and asked if they could find out why. The answer was cavivation erosion of the fillet radius of the crank. Hi oil pressure, and not enough rod side clearance was the problem. Ford want's .004-008", I run my street engines at .010-.012" on the rod and .015-.020 on the bearing for racing. We switched to Pensoil 40w Racing oil and Full flow oil filter system. Never lost an engine since. I have a pictiure of the crank some where. I don't think that was the reason for this failure, but we all learn by these problems, so it doesn't happen again.

[35cab]

Thanks for all the input and support and to Mark for photos.
Attached are three more close up shots of the break, not great pics, just from my phone.
Pic 1, notice small (old) hole? round that hole you might catch just a hint of a circular mark, almost looks like it has been plugged?
Next photo is the inside of that hole, note the surface, maybe evidence of repair?
Third one you might see at around 2 o'clock another small irregular hole.
Did people weld up cranks? I have no knowledge of this being dropped, it was re ground by a respected flathead builder (no longer trading) and I have every faith in him but I would have thought he would have picked up on this.
Interesting points about the radius.
As Mark pointed out it is a pretty standard motor, 4 inch crank, skimmed stock heads, plus 10 bore due to sticky piston rings but really just a refurbished engine.
I don't drive it hard but it has a columbia so I can and have sat at 65 -70 for long periods .
Not been aware of any sudden movement that could have put any serious shock or sudden twist so maybe just bad casting and bad luck.
In someways it was lucky as there does not appear to be any other damage, will have to check rods particularly the ones connected to that journal.
Gordon.

[Bulligen]

It was probably a flaw going all the way back to when it was cast. Just time and use and it finally showed up.

[big deuce]

Originally Posted by aonemarine

simplest answer.....its 70 frigging years old!

If its a French crank,it may not be near that old. My French block was cast in 1978, and components probably similar in time frame.

[J Witt]

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As Bulligen said it was probably a flaw that's been working for awhile. It doesn't take much of a flaw to create a stress raiser that will start a crack. That spiral shaped break looks like a classic torsion stress fracture to me. Sure would suspect too small a radius in the journal fillets.

Glad you didn't get any other damage from it.

John

[Tim Ayers]

OK, how bad of a flaw is too bad? One of my 4" has a tiny pocket on one of the journals. It was also re-done by a well respected builder/machinist and he said nothing to worry about.

After seeing this, I'm nervous.

[J Witt]

Size of flaw is not so important as the shape and location. Some of the general rules are (without making a failure treatise):

How sharp is the flaw? A scratch or notch with sharp edges causes a stress concentration across the sharp edge which can lead to stresses near or greater than the material yield. This is the reason for putting as large a radius as possible on journals and a radius on the oil hole drillings.

How high is the working stress around/over the flaw? Race engines have more stress than street engines.

Where is the flaw located? A variation on #2. A scratch in the oil passage is not as important as one on the journal surface. There is zero stress in the exact axial centerline of a journal, which is why that area can be drilled out.

How hard is the material? The surface of the crank is hardened which reduces the ductility of the metal. That in turn makes crack propagation more likely. Shot peening relieves the surface stress in a part and reduces the possibility of having scratches, nicks and notches. You can't shot peen the journals, unfortunately, but its often done to the outside of con rods and other stressed parts.

These are general statements. Real world failure analysis often devolves into testing for definitive answers.

Hope this is of some interest. I recommend Shigley and Mitchell's "Mechanical Engineering Design", Mcgraw Hill, if you want more info. This is an approachable handbook that is useful for the auto enthusiast. I still have my 1983 edition and refer to it. A used one is just fine, like Machinery's Handbook, the info doesn't change very fast.

John

[Tim Ayers]

Originally Posted by J Witt

Size of flaw is not so important as the shape and location. Some of the general rules are (without making a failure treatise):

How sharp is the flaw? A scratch or notch with sharp edges causes a stress concentration across the sharp edge which can lead to stresses near or greater than the material yield. This is the reason for putting as large a radius as possible on journals and a radius on the oil hole drillings.

How high is the working stress around/over the flaw? Race engines have more stress than street engines.

Where is the flaw located? A variation on #2. A scratch in the oil passage is not as important as one on the journal surface. There is zero stress in the exact axial centerline of a journal, which is why that area can be drilled out.

How hard is the material? The surface of the crank is hardened which reduces the ductility of the metal. That in turn makes crack propagation more likely. Shot peening relieves the surface stress in a part and reduces the possibility of having scratches, nicks and notches. You can't shot peen the journals, unfortunately, but its often done to the outside of con rods and other stressed parts.

These are general statements. Real world failure analysis often devolves into testing for definitive answers.

Hope this is of some interest. I recommend Shigley and Mitchell's "Mechanical Engineering Design", Mcgraw Hill, if you want more info. This is an approachable handbook that is useful for the auto enthusiast. I still have my 1983 edition and refer to it. A used one is just fine, like Machinery's Handbook, the info doesn't change very fast.

John

Thanks, John. Makes sense. I'll look for that book for sure.

[rheltzel]

Maybe it was stored horizontally.

[jake197000]

x2 aonemarine

[40 Deluxe]

Originally Posted by rheltzel

Maybe it was stored horizontally.

Yes, in a dark corner of the shop where the metal termites ate it out from the inside. Or the shop wall had a crack in it and the sun's UV rays destroyed the molecular structure.

[Dodge]

Lack of a Radius for sure. To put in a good radius you have to keep sharping the wheel.
Do the next one somewhere that does race or high performance cranks.
Also keep the next on under 9,000 rpm.......

[scooder]

Originally Posted by jake197000

x2 aonemarine

You did read the whole thread didn't ya?
Its most likely not that old, it's French flathead 4" crank. These were still being made in the early 1990's.
If it was an age related thing, there would be snapped crankshafts littering the floor of most of the folk on here.
Martin.

[bbrocksr]

Originally Posted by 35cab

Thanks for all the input and support and to Mark for photos.
Attached are three more close up shots of the break, not great pics, just from my phone.
Pic 1, notice small (old) hole? round that hole you might catch just a hint of a circular mark, almost looks like it has been plugged?
Next photo is the inside of that hole, note the surface, maybe evidence of repair?
Third one you might see at around 2 o'clock another small irregular hole.
Did people weld up cranks? I have no knowledge of this being dropped, it was re ground by a respected flathead builder (no longer trading) and I have every faith in him but I would have thought he would have picked up on this.
Interesting points about the radius.
As Mark pointed out it is a pretty standard motor, 4 inch crank, skimmed stock heads, plus 10 bore due to sticky piston rings but really just a refurbished engine.
I don't drive it hard but it has a columbia so I can and have sat at 65 -70 for long periods .
Not been aware of any sudden movement that could have put any serious shock or sudden twist so maybe just bad casting and bad luck.
In someways it was lucky as there does not appear to be any other damage, will have to check rods particularly the ones connected to that journal.
Gordon.

Looks like maybe that throw was welded up and reground. look at the crater and porous surface in picture one.
Bill