Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Un equal weight connecting rod will creat problem?

  1. Dec 22, 2011 #1
    Hi..all. I have replaced a connecting rod against a failed one (in a 10 piston Benz engine) which is 100 GM lesser than original one since the supplier has modified to Euro norms and old rod no more availble. Now after woking 100 odd hours, the engine failed. After dismantling the engine it was found, the oil pump found failed and metal particles found in the sump. Is it possible that the unequal connecting rods would have generated unbalenced dynamics in the engine and made the crank shaft bearings or con. bearing wear faster or make it fail?
    Sorry for the common man language!
  2. jcsd
  3. Dec 23, 2011 #2
    I don't know the answer to the question although I have ran my Honda engine with forged rods that are heavier than OEM spec and all seemed fine. Just my experience.
  4. Dec 23, 2011 #3
    Well you'll certainly get a vibration with unbalanced rods (ie one of them being significantly lighter than the others). Depends what the metal shavings are, and where they came from. 100 hours seems low for any engine, I'd put my money on the new rod being the cause of the engine failing.

    It could just be coincidence though, it's too hard to tell from what you said.
    Last edited: Dec 23, 2011
  5. Dec 23, 2011 #4

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    Traditional Quality Control standards state that connecting rod weight must not vary more than 9 grams in any given engine. That stated, you have a situation that is ten times over the normal proven process. Question - how many cylinders is the engine?
  6. Dec 23, 2011 #5


    User Avatar
    Science Advisor
    Homework Helper

    You really need to figure out what the failure sequence was. Did something in the crank case break up and metal chips blocked the oil supply and/or wrecked the oil pump, or did the pump fail first and the engine then failed (and wrecked a bearing) through lack of oil?

    Whatever, thinking about it from first principles, having different mass rods on cylinders with the same crank angle is going to cause unbalanced loads that the crank was not designed for. Whether you would get away with sets of different mass rods at different angles is not so obvious. It might lead to torsional vibration problems, for example.
  7. Dec 23, 2011 #6
    100 grams is almost a quarter pound. On something that is making 4-8000 strokes per minute (up and down strokes) that is going to create one hell of a problem. Now, you say that the oil pump failed? That's it? You found metal shavings, but nothing broken or excessively worn?
  8. Dec 24, 2011 #7
    While the weight difference will cause an imbalance and a vibration, it's highly unlikely that it caused the oil pump or the bearings to fail. The usual vibration induced failure is a cracked or broken part and bearings don't tend to fail unless the crank fails first, and even then I've seen bearings in good condition on either side of the break.

    Not aligning the pump so that the shaft and gears don't bind when rebuilding some Mercedes engines certainly will cause it to fail however.
  9. Dec 24, 2011 #8
    I would have thought binding would lead to instadeath of the box when you tried to put torque thought it.

    I think this will remain a mystery until there is a strip down. Would be intersting to see pictures.
  10. Dec 24, 2011 #9
    Depends how severe the bind is; on my old 617 diesel it was enough to be noticable but not locked up before I adjusted it. If it had been left as it was it would have resulted in excess wear and likely eventual failure.

    I had disassembled it to check clearances and wear and didn't quite have things in line. After the adjustment it spun smoothly and freely.

    You're right, I'd like to see some pictures too!
  11. Dec 25, 2011 #10

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

  12. Dec 25, 2011 #11


    User Avatar
    Gold Member

    I'm guessing one per piston, which would mean 10. (See bracketed section of the second sentence of original post.) :wink:

    Unless you own a 60's-70's Mopar 440. :rolleyes:
    They really don't like to oil the #4 rod bearing properly, and the results can be catastrophic. Been there, done that. Either Ranger or Stingray (sorry that I can't remember who) gave me a great link to a preventative manoeuvre involving some minor surgery. It was too late for me, but I now know why it went pear-shaped and can advise others.
    I agree, however, that 100 gm is a horrific mismatch. I was never anywhere nearly as competitive as Ranger or Stingray or Hypatia, but my piston/rod combos were balanced within a gram of each other.
    Last edited: Dec 25, 2011
  13. Dec 26, 2011 #12

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    Danger...I doubt the non balance thing led to catastrophic melt down..prob assembly situation..like maybe fatigued con rod bolt..or oiling..like we figured out with external plumbing cure a while back..
    Happy New Year..fellow mopar fiend
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook