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+--- Thread: The symptoms of a 3 bearing crank failure? (/showthread.php?tid=1985)
RE: The symptoms of a 3 bearing crank failure? - Ruairidh Dunford - 09-11-2018
That’s a belter Chris.
I must post up a photo of the rod that failed in our Twelve - the engine still ran. Will come back...
RE: The symptoms of a 3 bearing crank failure? - Terrytuned - 09-11-2018
Here are a couple of 2 bearing Phoenix 1 5/16" cranks that came out of race engines I would only advise an 1 1/2" crank for racing.
Terry.
RE: The symptoms of a 3 bearing crank failure? - Bob Culver - 09-11-2018
Hi KC
Your post appeared while I was still pondering.
It is not as classic as most. I am not sure where final fracture is. Usually crack starts at inside and works outward, but seems to have formed at outer diameter. Is it from an engine in normal use or high rpm?
RE: The symptoms of a 3 bearing crank failure? - Ruairidh Dunford - 09-11-2018
Those are spectacular failures Terry!
This rod was in the ‘26 Heavy 12/4 - we were returning to Bristol after a long weekend in Lake District. As we passed Gloucester on the M5, doing about 45/50mph, there was an almighty bang and tremendous vibration. My father pulled over and eventually we were relayed the last 25 miles home.
On stripping there was no sign of the piston - just thousands of tiny aluminium pieces.
The rod was in the sump and the engine still ran. This rod is about twice the size of an A7 one.
I always thought it would make a good novelty toolbox handle...
RE: The symptoms of a 3 bearing crank failure? - Reckless Rat - 09-11-2018
Just curious, why is the 1 1/2" crank more favoured for racing than the 1 5/16ths? Is is something to due to harmonic resonance or just less overall mass?
RE: The symptoms of a 3 bearing crank failure? - Chris KC - 09-11-2018
(09-11-2018, 08:25 PM)Terrytuned Wrote: Here are a couple of 2 bearing Phoenix 1 5/16" cranks that came out of race engines I would only advise an 1 1/2" crank for racing.
Terry.
Crikey
(09-11-2018, 08:40 PM)Bob Culver Wrote: Hi KC
Your post appeared while I was still pondering.
It is not as classic as most. I am not sure where final fracture is. Usually crack starts at inside and works outward, but seems to have formed at outer diameter. Is it from an engine in normal use or high rpm?
Must admit I've been re-examining it myself and wondering the same Bob. I'm no expert but I reckon it's 2 cracks which have met up in the middle. Evidence slightly obscured by the subsequent mashing together. It was an engine in normal use by a 20 year-old student <ahem>
(09-11-2018, 09:10 PM)Ruairidh Dunford Wrote: Those are spectacular failures Terry!
This rod was in the ‘26 Heavy 12/4 - we were returning to Bristol after a long weekend in Lake District. As we passed Gloucester on the M5, doing about 45/50mph, there was an almighty bang and tremendous vibration. My father pulled over and eventually we were relayed the last 25 miles home.
On stripping there was no sign of the piston - just thousands of tiny aluminium pieces.
The rod was in the sump and the engine still ran. This rod is about twice the size of an A7 one.
I always thought it would make a good novelty toolbox handle...
'the engine still ran...' Impressive!
It's the M5 triangle Ruairidh... mine went within pushing distance (you could get away with it in those days) of Taunton Deane services, luckily not so far that Dad couldn't come up and trailer me home. Apart from one occasion when I was still running but decided not to chance it, the only time she's not come home under her own steam (looking for wood to touch...)
RE: The symptoms of a 3 bearing crank failure? - Zetomagneto - 09-11-2018
(09-11-2018, 09:10 PM)Ruairidh Dunford Wrote: Those are spectacular failures Terry!Interesting, obviously the piston failed, usually it is the rods that fail in the 12/4 and result destruction is comprehensive
This rod was in the ‘26 Heavy 12/4 - we were returning to Bristol after a long weekend in Lake District. As we passed Gloucester on the M5, doing about 45/50mph, there was an almighty bang and tremendous vibration. My father pulled over and eventually we were relayed the last 25 miles home.
On stripping there was no sign of the piston - just thousands of tiny aluminium pieces.
The rod was in the sump and the engine still ran. This rod is about twice the size of an A7 one.
I always thought it would make a good novelty toolbox handle...
RE: The symptoms of a 3 bearing crank failure? - Ruairidh Dunford - 10-11-2018
The overwhelming opinion at the time was that the big end bolt had let go first.
RE: The symptoms of a 3 bearing crank failure? - fatcatvera - 10-11-2018
I’ve managed to get the engine out of the car and mostly apart this evening. The crank has failed across the web to the rear of the centre main bearing. Very similar to the image that Rauiridh has shared. It hasn’t quite cracked the whole way across the web yet - there’s still about 10mm of joined metal. I can imagine the mess it would have made of the crankcase if it had failed completely.
I’ll upload some images when I’m at a computer (I’m writing this from my phone).
Chris KC - it is indeed a small world! My brother told me he’s had a chat with you - have a you got a spare 3 bearing crank?!
Ray
RE: The symptoms of a 3 bearing crank failure? - Tony Press - 10-11-2018
Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.
He has now has a new engine with a two bearing crank.
John McMillans Crankshaft failure 1.jpg (Size: 52.07 KB / Downloads: 324)
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John McMillans Crankshaft failure 3.jpg (Size: 67.25 KB / Downloads: 321)
John McMillans Crankshaft failure 2.jpg (Size: 70.19 KB / Downloads: 320)