Piston broken in half(top-bottom) 5

[dirk489]

I reconditioned a friend's engine which broke later on during running in.The piston broke just on the pin line,conrod bolts snapped clean off, smashed the whole block.
My question is: is this always due to overreving or is there another possibility?
Dirk/Antwerp

 

[desertfox]

Hi

I have followed this thread with interest for quite awhile now and one thing that caught my eye is that the original bolts were torqued dry to 6.9kgm, well torquing bolts dry can leave a huge variation in bolt preload +/- 25%, so is it possible that preload variation allowed one bolt to see more of the load than the other?
If so this might well support the theory of the last poster who suggested one bolt failed first in fatigue and subsequently followed by the second bolt.

Just a thought.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dirk489]

The reasoning is right,with the exception that the bolts were new and engine only did 50 miles in total. I would not expect fatigue marks on an engine which is basically still running in?I gave the rod to the University lab and they will examine it next term. Will keep You posted then.
Dirk

 

[desertfox]

hi dirk

Thanks for the response.

I don't think something being new would exclude it from failing in fatigue and besides how many cycles would the piston do in 50miles, it may also be a small flaw in the bolt material itself which wasn't detected.
Anyway it will be interesting to see what the university says.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[tbuelna]

There is something called "low cycle fatigue". And 50 miles of driving could easily put 1x10^5 or more fatigue load cycles on the rod bolts. If the failed bolt had a defect, was over-stressed at installation, or was over-stressed during operation, it could easily have failed in low cycle fatigue.

 

[ICEaddict]

I have approx. 10 years in building engine's as a full time career at 5 different facilities covering oem rebuilds to 1000Hp forced induction applications. This would be the first time I've seen an over-tqd rod bolt cause a chain of events to damage a piston like that. I am curious how close the crankshaft counterweights come to the bottom of these pistons. Older engines had heavy piston castings and longer than average strokes. This is what mainly seemed to determine their connecting rod length, which varies a lot within engine models year to year and displacement sizes. This combination doesn't allow for much rod twist/bend, bearing play, or even part combinations before the bottom of the piston is contacting the crank counterweights and fracturing the bottom of the pin bores while in turn putting the rod bolts in excessive tension.
The only assumption I can make here is the rod bolts broke after the counterweights fractured the piston. The g-forces on a heavy piston are high and at TDC I'll bet the pin was pulled through the bottom of its bore after it was weakened. However the rod bolts broke after that is anyone's guess.

 

[ICEaddict]

Also did you measure the cylinder bore diameter and out-of-round? I can't see the thrust surfaces of the cylinder well in the block pic. Are they scuffed at the bottom as bad as the piston is near the second ring land?
The most difficult task with cylinder honing is preventing taper in the cylinder bore. Due to main webbing, the honing stones are limited on their travel beyond the bottom of the cylinder, and if the stones used had previously broken from contact on another job, taper becomes very hard to control. Many of the engines I've seen done by other shops have excessive cylinder taper, but this would be the first time I've seen a piston fracture as a result.

 

[flavioct]

This is not a con-rod bolt problem, it is only a typical problem with cast piston broken in pin support area.
With cast piston you cannot have the control of metal uniformity, you don't have control of how it was cast and how it was cooled.
I have 50 years in racing engine, racing, building engines and as tech scrutiner and I have seen many times this problem with cast pistons.
Use only forged piston, and rarely you will see a piston broken this way, they cost more but a broken engine like this cost many times more.

 

[dicer]

flavioct
I fully agree with you. Me too about 50 years playing with ICR engines of all types and sizes.

 

[Panther140]

I wish we knew if there were manufacturing defects, or had some data from the engine leading up to the events. Did you send them the other rods and pistons to be analyzed?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[flavioct]

The alloys "ageing" (example Al alloys) are structurally unstable, they are in a state of transition towards a stable state of equilibrium.
Consequently, in the conditions of sliding viscous, it is likely that the diffusion of atoms, increased by temperature and stress, involves the reactivation of aging process, resulting in a fast reduction of mechanical strength and brittleness.
Remember this cast piston is only 57 years old

 

[Panther140]

What kind of fuel do you run in an engine from 1958? If I remember correctly, thats a relatively high compression engine, is it not?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[dirk489]

To Panther 140:the rod and piston are at the University as a project,will get results in april/may.
Investigation of the other rods showed that the thread was torn out partially on several of them.
That can only happen when a very heavy pulling load was applied,which makes the overevving plausible.
These engines run at a 9.9 compression ratio,works just OK with 99 octane fuel ,30 ° advance.
The race engines I built have Cunningham conrods and Arias pistons,1680 cc giving 163 Din BHP at engine(120 KW)at 7500 rpm.
Dirk

 

[Panther140]

Interesting, glad they got back to you with more information. I wonder what sort of hardening/annealing processes (intentional or not) happened to the bolts throughout their life

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[dirk489]

Well,the proposed project wasn't taken up by one of the students. Pity, as the lecturer found it interesting.He will keep the parts ,maybe later on someone will take it up.
Thanks all for the interesting comments.
Dirk

 

[TrackRat]

I'd be very interested to hear what the University determines in this matter. I am not a metallurgists nor do I pretend to be one on TV... but a number of years ago I had an opportunity to examine some (new) connecting rod bolts that were sheared off within minutes of starting a recently rebuilt engine. Several metallurgical labs examined the parts as well as a University. As indicated by someone above, each independent reviewer knew immediately that the bolts failed due to impact shear. I believe they refer to the "image" or "signature" on the broken pieces as a "thumbnail". Even though the engine builder claimed the bolts were "new defective", the impact signature and no metallurgical defects in the fractured bolts, strongly suggested the tightening torque was insufficient allowing the bolts to loosen and impact the block, shearing them off.

In regards to the 55 ft/lbs. tightening torque, normally I too would agree that 55 lbs. is excessive for a 3/8-24 thread bolt, however there are many variables and that is why when possible bolt stretch is used to preload the bolts instead of rotational torque, which is simply a measurement of rotational resistance, not of clamp force/tension. Obviously in this case using bolt stretch is not possible with a blind hole. It is possible as noted by one person here who modeled the bolt, that 55 ft/lbs. would not exceed the stated tensile strength or hopefully the yield strength unless designed to do so. As much as 60% of the tightening torque of a lubricated bolt can be required merely to overcome the friction between the threads and bolt head contact surface. Thus a relatively small percentage of the tightening torque ends up tensioning the bolt. In addition slippery lubes like moly / cam lubes can require as much as 30% lower tightening torque than regular petroleum engine oil as they lower the coefficient of friction. Since the factory specifies 55 ft/lbs. torque on these oiled bolts, assuming an accurate torque wrench was used, then there is no reason why 55 ft/lbs. should have caused the bolts to fail. If the bolts were over-tighten I'd expect to see necking down of the shank/threads at the fracture, which doesn't appear to be the case from what I can see in the photos.

Unfortunately cast pistons have very unpredictable durability in many applications. In determining which came first the broken piston or the broken rod bolts, I'd be willing to venture the piston failed, blocking the rod and shearing the bolts off with the clear "signature" pattern that metallurgist all seem to recognize. It should be interesting to see what the University experts are able to determine.

 

[ukgh]

Hi,

Very interesting topic.

On these older engine the pistons, con-rods & caps, were just put together, off the assembly lines bench tested off into cars.
During my time racing and tuning in 60's, every engine we rebuilt we balanced, the piston, the rods and cap separately, you be surprise how far these could be out. a very simple method is balance scales. interesting to check the remaining piston rods.

One question not asked, whether you know or not was the engine cold? not up to running temperature, remembering these are high tensile but brittle when cold. A combination of cold engine, unbalance pistons ect (

I also support the theory of a loose bolt, or something could have given cause to loosen slightly. What you don't notice when stripping an engine and rebuilding engines is, and many people fail to notice is the tiny lip you get on the faces of both the con-rod and cap and main bearing housing and cap. Its always a good idea to run a file over the faces and deburr the edges. found this out using plastigauge. I believe you can still get it.

Probable the drivers had a heavy right foot, when rebuilt put a rev limiter on.

Hepe you resolve this.

Regards

 

[dirk489]

Engine was rebuilt with Arias pistons and indeed rev limiter!It was difficult to find decent rod bolts as not covered by ARP(Too long)
Dirk

 

[gruntguru]

Do you mean original rebuild or subsequent?

je suis charlie

 

[BigClive]

I am sure I read somewhere that the MGA Twin Cam engine was prone to failure after being rebuilt - don't know why that would be.

 

[Panther140]

Has anybody here ever encountered an out-of-round crank pin? Lets say you had a flat spot on your crankshaft and then torqued your perfectly round brand new con rod onto that. Think about the effects of that as the crank pin rotated within the con rod. It could be extremely tight at certain angles.

Was this a new crank and rods, or was the same crank used and new rods/bearings/bolts? The reason for rebuild? Metals can deform and that engine was known for knocking.



"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin