snaped crank shaft

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kieronriley

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what could make the crank shaft on a petter mini twin snap.
the engine had only done app. 40 hours from new,it had been in storage from new.unsure of how many years that was,but it had a full sump of oil and so far as I am told it was not running hot,and after the initial bang and engine stoping it was turned over freely though be it with a rattle,so what do you think could have been the cause.i am about to buy the engine as I have a brand new crank shaft in my stock of petter parts,but would appreciate your thoughts.thanks,kieron
 
Interesting one, where did it snap?
Does the crankshaft have internal oilways? (I've never delved below the base gasket on one of them) because despite oil in the sump, perhaps it rusted from the inside out.
Perhaps it was hydraulic lock, water passage failed and let water on top of a moving piston on the compression stroke?
Perhaps whatever the engine was driving came to a sudden stop.

A jolly detective story let us know the answer!

ps...
I don't think dropping a valve could do it, but maybe.

A tool left in the crankcase?
I would strip it down for a very good look, only takes a few minutes on them.
 
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hi the engine does have internal oilways,and it was driving a boat at its demise.it has snapped at the big and of no. one piston,and does not seem to have dropped a valve or had a hydraulic lock.kieron
 
It takes quite some force to crack a crankshaft! I'm guessing either a bearing siezed solid or the shaft was defective. My second guess is that you will find plenty of evidence of the cause when you strip it and it may well be that being rebuilt with everything freshly oiled you won't have a recurrence of the problem.

Rob.
 
Agree there.
Strip it. If there is no secondary damage then it was a faulty crank. If something caused a sound crank to bust there will be plenty of evidence. Bent conrod, sheared bearing cap etc.
There may be scuffs and gouges from the loose bits, but to cause a crank to bust there needs to be some pretty major forces at work.
 
It sounds like it could have been a flaw in the forging. If it had been stored for years then moisture could have got into any cracks and made them worse. I have also seen cracks in the crank pins caused by wrongly matched auxiliaries placing too much stress on the free end of the crankshaft.
 
A sure way to break a crankshaft is to ommit the radius from the big end or main bearing journals to the crank webs.That will create localized stresses that will result in breakage.Maybe the crank had been badly ground.
 
kieronriley said:
what could make the crank shaft on a petter mini twin snap.
the engine had only done app. 40 hours from new,it had been in storage from new.unsure of how many years that was,but it had a full sump of oil and so far as I am told it was not running hot,and after the initial bang and engine stoping it was turned over freely though be it with a rattle,so what do you think could have been the cause.i am about to buy the engine as I have a brand new crank shaft in my stock of petter parts,but would appreciate your thoughts.thanks,kieron
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until you strip it down you wont know for sure but I have seen crank bearings eaten away by electrolysis in engines that have been stored for years. That said a faulty crank is my first suspicion.
 
Loose flywheel? (Especially if the output and flywheel are at opposite ends of the crank - I don't know if that is the case with this engine.)
 
30boat said:
A sure way to break a crankshaft is to ommit the radius from the big end or main bearing journals to the crank webs.That will create localized stresses that will result in breakage. Maybe the crank had been badly ground.
Click to expand...

I have seen many failures like this and actually suffered one myself, Austin Healey Sprite with BMC A-series engine. That particular crankshaft had never been reground, although as you say many fail due to being badly reground. In these examples the cause was fatigue. If the fracture is found to be smooth and polished, with no other damage, when the Petter is stripped, that will be it.

When I broke mine I drove 10 miles into work and 20 miles home again. Made a dreadful knocking noise but barely lost oil pressure. The crank had broken on the back main bearing, next to the flywheel.
 
The crankshaft snapped on my old 2.8 V6 "Cologne" Granada engine at about 100,000 miles. I replaced it and fitted new (white metal) bearings, and it ran perfectly for years (until the floor pan snapped in half :o. ) I never understood the reason for the crankshaft failure. I guess it there must have been a crack or some kind of defect from new.
 
kieronriley said:
what could make the crank shaft on a petter mini twin snap.
the engine had only done app. 40 hours from new,it had been in storage from new.unsure of how many years that was,but it had a full sump of oil and so far as I am told it was not running hot,and after the initial bang and engine stoping it was turned over freely though be it with a rattle,so what do you think could have been the cause.i am about to buy the engine as I have a brand new crank shaft in my stock of petter parts,but would appreciate your thoughts.thanks,kieron
Click to expand...
could be the gear end camshaft gears, oil pump gear and so on, there's plenty to go wrong. Has the crank case been damaged you will need to strip the whole engine because a major fault like that can cause other problems

mick
 
Mazda MX5s had a period of crankshaft breakages. It affected the pre 1990 1600cc models (the more powerful 115bhp one).
It only happened after the crankshaft damper had been removed and replaced as part of other repairs. The theory was that the bolt torque was wrong. Either too tight (bolt stretch) or too loose and the keyway would chatter and vibration set in.
 
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thanks very very much for all of your help and advice I will be collecting the engine on Friday and will try to start work on dismantling over the weekend.but it may take a little longer to start as I have a few family birthdays and I think I will be shamed into attending rather than working on !that bloody old engine,so I will write and let you all know what I find,and maybe seek more advice.so thanks again.kieron
 
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Some years ago my daughter arrived home from work in the Mini Metro we had owned from new and told my it was making a strange rattling noise.
Into our local garage where they found the crankshaft snapped in half.I took a look and was suprised to se that it was still in situ in the main bearing but that you could slightly turn one half against the other.
We agreed to replace the crankshaft and car then ran perfectly all right until rusting to bits some six years later.
When I examined the old crankshaft there was clearly a crack in the forging which had I think been there from new.
 
BMC A-series crankshaft. Fatigue failure initiated at two origins, one at each side of the oil hole.
Crankshaftfatigue_zpsae0d48dc.jpg
 
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