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Looking for opinions on connecting rod failure

  1. Oct 26, 2010 #1
    Unfortunately the piston has disintegrated so we are unable to use that to form an opinion on this failure. Please let me know your thoughts, thank you!

    [PLAIN]http://www.maperformance.com/gallery/pictures/rodfailure1.jpg [Broken]

    [PLAIN]http://www.maperformance.com/gallery/pictures/rodfailure2.jpg [Broken]

    [PLAIN]http://www.maperformance.com/gallery/pictures/rodfailure3.jpg [Broken]
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 26, 2010 #2
    Need some more info about what was happening at the time. Any previous history of issues?
  4. Oct 27, 2010 #3
    This is an AWD automotive application and the failure occurred while drag racing during the initial launch / first gear acceleration. I'm basically trying to gather input on whether the piston or the connecting rod was the cause of the failure, or if it is likely that the engine was over revved, experienced detonation, etc.
  5. Oct 27, 2010 #4
    Well, the first two pictures are a bit disorienting to me, but the third picture is more clear. Looks like a classic case of a bent rod due to the piston contacting something solid, either a valve or a piece of the piston itself.
  6. Oct 27, 2010 #5
    The rod is actually bent into a "U" shape. As you can see the wristpin is still intact and the connecting rod has been folded into this orientation within the crankshaft counterweight.
  7. Oct 28, 2010 #6


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    I can say from personal experience that if you're running a 440 Mopar, your #4 rod bearing wasn't properly oiled. Ranger Mike gave an excellent account of the cause and cure for that in a previous thread. He's the "go to" guy for automotive stuff. Stingray used to be very knowledgeable as well, but I haven't seen him around in quite a while.
  8. Oct 28, 2010 #7
    This is actually a 4 cylinder Mitsubishi application and I forgot to mention that it is making about 600whp. The pistons and connecting rods were upgraded forged units as well.
  9. Oct 28, 2010 #8

    Ranger Mike

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    My bet is detonation ..but here are some things to think over..
    The connecting rod connects the pistons to the crankshaft. It converts the linear motion of the pistons to the rotary motion of the crankshaft. On every stroke, the connecting rod is stretched and compressed. This pressure, plus other factors, can cause the connecting rod to break.

    The number one cause of failure is fasteners followed by rod breakage. Fasteners are the weakest link of the con rod. There are two types of fasteners. Through bolts and cap screws. The through bolt requires a nut. a flat must be machined into both the rod and end cap and this breeds stress risers. A capscrew threads into the big end and helps strengthen the whole cap/rod scene.

    As I said above , Fatigue is the main cause of broken connecting rods. The constant compression during the power stroke and stretching during the exhaust stroke, over thousands of times a minute, eventually wears the metal out and it becomes brittle and finally breaks at the inclusions or stress risers on the rod surface, These are caused by the forging process..its like putting a nick or notch on a coat hanger and bending it until it breaks.If the oil is low or dirty it can speed up this process. Running the engine hot can also speed up the process.

    The pin that connects the connecting rod to the piston (called the piston pin, wrist pin or gudgeon pin) gets a lot of wear. If this pin snaps the connecting rod is no longer connected to the engine. For some engines this results in catastrophic engine failure--the connecting rod goes through the engine block or the crankshaft is bent. Piston pin seizure is a common cause of connecting rod failure. It the rod bolts/nuts are still intact the pin probably seized in the piston bore.

    Over revving is the main cause of connecting rod failures in new and high performance engines. If the tachometer hits the red--even briefly--the connection rods are in danger of breaking. This is because the forces acting on a connecting rod increase dramatically at high revolutions. It does not matter if the tachometer is going into the red because the race car is traveling at a high speed, is going too fast in a low gear or is simply going too fast because the accelerator is pressed too far while the car is in neutral--the stress is simply too high at extremely high RPMs. One thing that may happen when you over rev is valve float to the point the head of the valve gets smacked and bent and breaks off, gets eaten by the piston and there goes the top of the piston. Then you have rod slapping around the bore. Not good. Generally the rod will be in one mangled piece more or less.

    Hydrolock is a deformation of the connecting rod caused when water gets into the piston chamber. This usually happens when the head gasket blows and water enters the combustion chamber. Even a momentary hydro lock will bend the rod and cause it's failure.

    One sure way to ruin an engine is DETONATION which would distort the rod bearing and cause it to spin and seize the Big end . Detonation will cause piston damage, bent rods, broken blocks, and bearing failure. I have never seen a case of detonation where there was not evidence in the combustion chamber.

    House keeping or lack of proper maintenance will ruin an engine as the table below ( from Clevite Bearings) shows. You plug or restrict oil and the bearing fails . This usually results in a lot of noise but no catastrophic damage..usually..

    Lets look at the rod bearing and Major causes of premature Bearing failure


    Misalignment ..........................12.6%

    Insufficient Lubrication.............11.4%

    Overloading ..............................8.1%


    Improper Journal Finish............3.2%Other ........................................2.8%

    Further reading = see
    Tension and Compression in Connecting Rods , A Failure Analysis by Luke Schreier , EM 325H , April 26, 1999

    Attached Files:

  10. Nov 9, 2010 #9
    Looks like a 4g63 to me. You said you were launching in 1st gear? What is the rev limit set at? did any of the other valves contact any pistons? (timing belt lash on a hard launch) It really looks more like the whole mixture in that cylinder lit off way too early. mechanical failure would more likely break the rod. was the cap still intact when you removed it?

    Looks like an Eagle I-beam or Crower?
  11. Nov 9, 2010 #10
    How much boost did it see? I'd have concur with awdgsx and say that it looks like the compressive forces exceeded the rod's capacity in order to bend it like that, especially given little or no bearing damage. How aggressive is your anti-lag system? Any possibility of ignition cross-firing? Detonation usually destroys the piston first but if the timing was way off with high boost ...

    To answer one of your questions, I'd say that the rod bent double on the compression stroke and the piston was destroyed by the subsequent contact with the crankshaft counterweights on the down stroke. Check for aluminum smearing on the counterweights and the lower edge of the cylinder. I don't see signs of lubrication failure on the rod bearing or the piston pin.
    Last edited: Nov 9, 2010
  12. Nov 9, 2010 #11
    Mender I agree for the most part but boost alone couldn't do that. I've run 50+ psi on methanol without any problems. Defiantly looks like anti-lag gone wrong.
  13. Nov 9, 2010 #12
    He might have had spikes well above what registered on the gauge. Depends how the ALS was set up and how well the car hooked (AWD) - could have dropped the launch rpm too much and caused a huge rise in boost as a result. Sort of the opposite of turbo lag. Then autoignition on the compression stroke, with a huge rise in pressure, bending the rod. Could have started the bending failure on one stroke and finished it off on subsequent strokes as well.

    Another thought: any sign of scoring on the walls?
    Last edited: Nov 9, 2010
  14. Nov 9, 2010 #13
    True but even cutting the rpm in half would only roughly double the boost. Most well set up cars are not going to leave with much more than 5psi, especially a stick car. At 600hp his is probably running around 35psi full tilt and there is no way he is making it before 2nd gear without nitrous. Either a huge chunk of the piston broke and wedged or severe detonation. There are very few things that will bend a good rod like that. It looks like hydrolock more than anything, even though I doubt that's what it was.
  15. Nov 9, 2010 #14
    I've read that aggressive ALS numbers can be up to 20 psi, but you're right, that alone shouldn't have caused the failure. I can't tell if there are any marks/signs of impact/wedging on the cylinder walls by the pictures.

    ETA: I edited out my comment about too much spray after a moment's thought and before I saw your next post - yes, that much would have a bad outcome.
    Last edited: Nov 9, 2010
  16. Nov 9, 2010 #15
    No way, that amount of spray would have exploded the engine. We just need more details..
  17. Jan 20, 2011 #16
    I’ll give a little info on here. This is my car it s 2005 evo 2.0L with Map/Howard rods, and JE pistons.
    2 years ago I installed the rods, right after 1000mi rod number 1 twisted right on top by the pin only making about 530 hp with a pte6262 turbo. I pulled the motor and found the twisted rod and sent it back, they sent me only one new rod, not 4. This one new rod doesn’t have the logo “howard” stamped on it, but I didn’t think anything of it at all at first.
    Motor goes back in the car with a 42R on it this time making 733awhp and like 400ft tq I know low *** tq. The 42r stayed on the car for about 500mi. Then got a FP black the car made only 4 to 5 dyno pulls making 600awhp and 580ft tq, by like 4500rpms with the fp black.
    So I pulled the car off the dyno hit the 2 step, let the clutch go and boom. I'm sure the rod or rods already got bent on the dyno just a little 2 step took them down.
    2 step at about 6000rpms, about 10 psi, and it blew up before it hit the rev-limiter at 8500rmps. I'm 100% positive that it was not over revved.
    I pulled the motor out to find a piston gone and a rod around the crank. Even the cylinder wall is 100% mint, valves never touched the pistons at all. Then I looked closer and I found 2 more rods that are slightly twisted right on the top by the pin. The only one rod that did not twist is the one replacement rod they sent me lasy year. I look even closer and it looks like the replacement rod is a little darker in color also, maybe diffident material who knows.

    All the bearings look mint, the head gasket was good, valves never hit, t-belt was good, tune in the car is prefect. My crower rods on my 1g are still holding 1000hp for 7 years + and still have never done this. Same goes for my 800hp gvr4.
    Please let me know what can cause this.
    Thank you
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