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Combustion connecting rod failure (in cars)

  1. Sep 28, 2010 #1
    Hi all,

    I have this question from my friends and hope get the answer.
    Using your car with the oil is not sufficient, the connecting rod is the first to fail. Normally it burns both bearings or the smaller one (connecting to piston) burns first?
    I think there's a dilemma here: the bigger one is lubricated better and not as hot, but it runs around more (more friction) than the smaller one.

    Thanks for any input.
     
  2. jcsd
  3. Sep 28, 2010 #2
    Lack of oil shouldn't cause a conrod to fail mecanically, i'd expect the engine to just grind itsself to a halt and seize up long before anything physically breaks.

    I suppose it depends on how the conrod failed. If it split in two and the piston came shooting out of the bonnet, then thats obviously not an oil problem. I suppose if the lack of oil caused the piston to weld itsseld to the conrod, then you have a huge amount of bending when the crank tries to rotate it.
     
  4. Sep 28, 2010 #3

    Ranger Mike

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    motor oil does two things, lubricate and cool. Pressurized oil squirts out of the con rod bearing onto the bottom of the piston and splash lubes the wrist pin. This oil carries away a lot of heat and cools the piston. You have 1200 degrees F heat separated by .200 inch of aluminum piston top. If oil galleries become clogged to the degree that oil is limited or even cut off, the piston will grow thermally and seize in the cylinder bore. when this happens the connecting rod bolt breaks and we have a catastrophic failure.

    If the oil system has not been modified to keep up with the performance modifications the same above failure will occur due to oil starvation at increased RPM. Usually the valve train suffers and you wipe out a camshaft lobe or you wipe out a main bearing or con rod bearing before the con rod fails but it is dependent on each situation.
     
  5. Sep 28, 2010 #4
    Thank you xxChrisxx.
    But we can not say the small bearing or the big one is more vulnerable if the oil is insufficient.
    Or by other way, after many years in use, which one 'goes' first?

    (sorry for my bad English)

    Thanks Mike,

    It seems clearer now.
     
    Last edited: Sep 28, 2010
  6. Sep 28, 2010 #5
    To be honest with things like this, Ranger Mike is the man to ask.
     
  7. Sep 28, 2010 #6

    Ranger Mike

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    the small bearing is not even a "bearing"
    the aluminum piston has a wrist pin bore
    the con rod has a similar diameter and the wrist pin diameter is the same. con rod " small end" is heated to expand and the cooler wrist pin is slipped through the piston and pushed through the small end of the connecting rod. it is a shrink fit. the con rod has a small diameter hole bored through the top of the small end and wrist pin bore. oil drips through this hole to lube the steel to aluminum faces. rarely will this end fail since the oil that is squirting past the con rod bearings is of great volume. this is why the con rod bearing side to side clearence must be within tolerance. too much clearence will starve bearings down line and too little will not lube the piston properly. there is a whole lot of squirting going on at 50 to 60 psi.

    typically, you will spin a con rod bearing (on the "big end") or wipe out the bearing material on the crank journal of the con rod. this will result in a tick tick tick noise deep in the engine.
    thanks Chris.....it is always great to have insightful advice as you do!!
     
  8. Sep 28, 2010 #7
    The big end is more vulnerable as it depends on pressurized oil flow to keep the bearing material and the rod journal (crankshaft) separated. The small end is lubricated by splash and only oscillates in the piston pin bore.

    The usual reason for a failed rod bearing is insufficient oil pressure for the conditions. For example, the rule of thumb was to have 10 psi for every 1000 rpm that the engine was to be subject to. If the engine was expected to survive 7000 rpm, 70 psi was the magic number. Failures usually happened when the oil got hot and thinned out, causing a drop in oil pressure and allowing the bearing to contact the rod journal with the inevitable result.

    Nowadays better oil quality and engine design allow that rule of thumb to be more like 7-8 psi per 1000 rpm but as Mike said, the clearances must be checked and held to carefully. There is a balance between pressure needed (tighter clearances require less) and sufficient oil flow between the crank and bearings for cooling (higher engine loads require more cooling, so require looser clearances).

    When done right, there is hp that can be freed up by the brave engine builder.
     
  9. Sep 28, 2010 #8

    Ranger Mike

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    When done right, there is hp that can be freed up by the brave engine builder.
    with deep pockets!!
    good point mender...thats why the builders went to honda bearings instead of those humongous size big block bearings we used to run...whole lot of friction and parasitic drag...plus metalurgy came a long way and synthetic oil replaced fossil oil ,
     
  10. Oct 2, 2010 #9

    brewnog

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    Under oil starvation, the connecting rod is actually usually the last component to fail (except for the cylinder block!)
     
  11. Oct 7, 2010 #10
    a quick doubt......... if there is insufficient oil to begin with, won't the engine get overheated and start knocking???? or maybe just blow out????? and if this does happen it would happen long before any mechanical damage wont it???
     
  12. Oct 7, 2010 #11

    Ranger Mike

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    lack of oil means metal to metal contact .it also add to engine heat because it is not carrying away heat from the combustion chamber ( cylinder walls and bottom of piston) , this makes detonation a lot more easy and detonation causes " knock" or pre ignition..the fuel air mixture pre ignites ( lights off before the spark causes ignition where it can be effective. ) this bends con rods, can melt pistons, break off chunks of piston land between the rings, and is an expensive situation. Back when we ran 13 to one compression ratio the cylinder would blow out if we had too much detonation..as a minimum you will blow a head gasket..maybe lead to blow out a head gasket water passage seal and now you have water flooding into the cylinder and we all know , boys and girls..H2O DOES NOT COMPRESS..so something has to give...
    it usually is the vacation fund you had saved up....
     
  13. Oct 7, 2010 #12

    Danger

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    "Knocking" usually indicates impending rod failure due to a bottom-end bearing failure. It can give you plenty of warning, such as a couple of weeks, or it might occur mere seconds before catastrophic failure. That's what happened to my Roadrunner about a year after I bought it, while I was on vacation. For some reason or another, 440's don't particularly like to adequately oil the #4 rod bearing. So there I was happily tootling along... the bearing spun out, leading to the piston immediately collapsing, which of course took the bore with it. Naturally, the bore was messed up enough that honing would have been inadequate, so a 30 overbore was necessary. Since the pistons and rods had to be replaced, I figured that I might as well do it up properly. So... it went into the shop in its original 400hp trim, and came out with 600. I since then added Blackjack headers and an Edelbrock CH4B intake to kick in an extra 50. Unfortunately, the cam is totally mismatched. Next step, if I ever get a chance to do it, will be a Mini-Express or Street Hemi grind. That should up the hp to around 800.
    Anyhow, back to the point. In my experience, there are 3 types of con rod failure. One is of the type that I just outlined. The second is cap bolt failure, the result of which depends upon what position and in what direction the rod is moving at the time. If it's heading upward, it goes through the block. If downward bound, it exits via the pan. The latter is far preferable. The 3rd possibility that I'm aware of is "rod stretch", wherein the elasticity of the rod is such that at high rpm's it can elongate enough to introduce the piston to the valves in a less-than-friendly fashion.

    edit: Just saw your post, Mike. I hadn't thought of pinging damaging the rods, but it certainly makes sense. I've seen it blow holes in pistons, but never extrapolated that to the rods.
     
    Last edited: Oct 7, 2010
  14. Oct 7, 2010 #13

    Ranger Mike

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    Danger..been there done that..
    Big block 440 s have stock external oil pump. the pick up is at front of the oil pump sump. Fine for granny and grocery getting but when you romp on it, oil sloshed to the rear of the sump and the pick up gulps air..not good for bearing..So we add the 1/2 ID Hemi oil pick up and go to deeper sump. One problem is that the oil runs through the oil galleries that are partially blocked by the lifters. As rpm increases oil is churned to the point it starts to foam..( air bubbles and oil mixture) this causes drop in oil pressure ( locally) and created air gaps. This is the same gallery that oils the crank bearings and finally the rod bearings.The oil has to pass through 4 lifters to get to number 4 rod bearing and is moving too fast to make the 90 degree turn. The reason the number 5 , 6 ,7, 8 bearings don't starve is because the oil backs up when it hits the narrower oil passage going to the valve train. The small block Chevy boys plug this gallery and drill a smaller id hole to keep the oil down stairs.
    To fix this problem we run a external braided oil line from the oil pump to the left rear of the block where the oil pressure sender usually is. This cures the problem by pushing non aerated oil to the galleries from both ends. This way you have oil coming in from both ends and meeting in the middle.
     

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  15. Oct 8, 2010 #14

    Danger

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    Wowzers! Thanks, Mike. I never had access to any fancy books, and this was long before the internet, let alone home computers, existed. This is the first explanation that I've seen for why those bearings go bad.
     
  16. Oct 8, 2010 #15

    Ranger Mike

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    yea Danger..I ran this set up on my 68 Road Runner..493 cid block..ihad a remote oil filter that had an extra oil outlet and I tapped out the gallery on back of the block and ran an Aeroquip line to it.
    for the price it sure can not hurt!
     
  17. Oct 8, 2010 #16

    Ranger Mike

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    here is my engine with hemi pick up and windage tray also note piston has minimum weight, stock 440 rods that are lightened
     

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  18. Oct 8, 2010 #17

    Danger

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    Cool, Mike.
    I don't know what sort of pick-up I have, but it's obviously extended since it came with the 8-quart Moroso deep-sump pan. I seem to recall that it's free-swinging, and the pan is baffled. The windage tray is factory stock.
    The pistons are "race-only" 12:1 aluminum TRW's mounted backwards in the bores and holding 1/16" double-moly rings with .008" end gaps. The rods are also TRW's that came with the pistons. Rod and main bearings are CL77's. Although there was nothing wrong with the original crank that couldn't be machined down, I chose to replace it with a forged steel unit which I think has chamfered oil holes.
     
  19. Oct 9, 2010 #18
    Minimum ring gap for that bore on the street should be .020". Tighter than that and you risk butting and breaking the rings.
     
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