Connecting Rod Failure in 4 Stroke Petrol Engines

[mrajm]

hi..
I am new to this site.
I want to know about reasons of failure of connecting rod of 4 stroke petrol engine. In this case the rod has broken into two parts from the small end of the rod. The manufacturing root of the component is hot forging - hardening - induction softening of the small end part - bend/twist correction - machine. What can be the possible causes of the failure?

 

[Ranger Mike]

see this thread - Oct26-10, 11:06 PM #1
ChrisCarey

Looking for opinions on connecting rod failure

 

[brewnog]

Did the piston seize? Small end failure is very unusual unless it's consequential.

 

[mrajm]

Did the piston seize? Small end failure is very unusual unless it's consequential.

On the part which remained undamaged in the block there is no mark of rubbing or seizure as such. Your point that it is quite unusual is correct as we have only received 6 cases out of around 5 lack engines we have produced, The big end is well in tact and appears to have no signs of improper lubrication,

 

[Ranger Mike]

i would say that the small end broke due to fatigue or a flaw. was the rod magnafluxed for cracks? was it inspected for cracks? I think the small end snapped because the piston did not move and the crankshaft did and therefore the con rod broke...now why this happened is known...maybe there was a blocked water passage around this cylinder..maybe the crankshaft was not oiling the bottom of the piston properly, maybe that cylinder had a lean fuel/air mixture. the rod , if in tact , without cracks should never have failed.

 

[Danger]

I agree that it was most likely a manufacturing flaw or fatigue, but is it possible that the thing was over-revved so badly that the "rod stretch" became infinite?

 

[mrajm]

i would say that the small end broke due to fatigue or a flaw. was the rod magnafluxed for cracks? was it inspected for cracks? I think the small end snapped because the piston did not move and the crankshaft did and therefore the con rod broke...now why this happened is known...maybe there was a blocked water passage around this cylinder..maybe the crankshaft was not oiling the bottom of the piston properly, maybe that cylinder had a lean fuel/air mixture. the rod , if in tact , without cracks should never have failed.

Thanks for the response..
Ok.. All the produced cam shaft go through magnaflux inspection as it is forging root. The possibility remains that the crack was not exposed after machining. There is no visible scoring mark or sign of seizure between the gudgen pin and the small end so for now we can rule out lack of lubrication. I will check the other two points and come back to you...
I will also try to provide photographs of failed surface so that something can be concluded on hardness, crack etc.