Symptoms of Connecting Rod Failure

[Joel T.]

Guys;

Aside from the obvious when a rod lets go, what are the classic symptoms of connecting rod failure.... i.e. a spun bearing or a bad piston pin (or anything else for that matter).

I'm trying to help someone diagnose a potential problem with a small block. The symptom thus far (and I have not heard it personally) is a sound like marbles rattling around at idle... which comes and goes..

Any information will be greatly appreciated.

Thx,

Joel

[Domenic T.]

Re: Symptoms of Connecting Rod Failure

Could be in the valve train. I listened to some hydraulic lifters that would bleed down at idle that made a rattle till the oil pressure rose. Also low oil pressure will cause the bottom end to do the same.
It's not synthetic oil is it? Also I asked about noise level tests back in the old days when multi weight oils became popular. They were higher than straight oils. That says something.
See what he has for oil, it could be a multigrade that's not right for his engine. There are a lot of areas to concider but the fact that it is at idle rings a bell.

DOM

[Clem Z.]

Re: Symptoms of Connecting Rod Failure

Guys;

Aside from the obvious when a rod lets go, what are the classic symptoms of connecting rod failure.... i.e. a spun bearing or a bad piston pin (or anything else for that matter).

I'm trying to help someone diagnose a potential problem with a small block. The symptom thus far (and I have not heard it personally) is a sound like marbles rattling around at idle... which comes and goes..

Any information will be greatly appreciated.

Thx,

Joel

with engine at idle short out one spark plug at a time and see if the noise goes away. you can use a plastic handle screwdriver to do the shorting out. DON"T TOUCH THE METAL PART of the screw driver. taking the load off of a bad bearing will most of the time quiet it down.

[Joel T.]

Re: Symptoms of Connecting Rod Failure

Dom / Clem;

Thanks! I'll start checking...

Joel

[Edward J.]

Re: Symptoms of Connecting Rod Failure

Joe, sometimes if you pull the dip stick and wipe in on a white paper towel you may see some of the rod or main bearing material, of even look at the oil on the dip stick if you see shiney pieces of metal suspended in the oil, you can suspect bearing material, lastly the oil filter to will show the same if removed.

Clems test will also work on rod bearing or the wrist pin bearing failure. but which one it is? sometimes wrist pin noise will lessen as the engine warms up, connecting rod noise will stay steady.

[Dan D.]

Re: Symptoms of Connecting Rod Failure

I've always tested for connecting rod noise by 'tickling' the throttle. That is with no load on the engine just slightly increase the throttle. A loose connecting rod will rattle as the RPMs increase. -Dan-

[Stuart F.]

Re: Symptoms of Connecting Rod Failure

One outside possibility is that of a broken crank at a main journal. When they break, they often have a third piece (sort of like a wedge) in between the two sections. The engine will remain relatively close to "in time", but the clearances between the pieces will increase causing some loss (lowering) of oil pressure and enough effect on timing to get slightly late or pre-ignition in several cyliders causing the rattling sounds. I've seen three back in the late 50's, early 60's.

Stu Fox

[Joel T.]

Re: Symptoms of Connecting Rod Failure

Guys;

My assumption is that once I figure out that we have a rod issue and hopefully zero in on one cylinder... this should be repairable.. We are probably looking at complete tear down and probably a refinishing of the crank.... and perhaps other things as well depending upon what I find.....

Joel

[Dan D.]

Re: Symptoms of Connecting Rod Failure

My experience with broken cranks that still drive is they will vibrate.
It does not take much to damage a crank. I would not run the engine very long until you diagnose the problem. I would not turn the crank. If it requires turning, I would replace it. -Dan-

[James H.]

Re: Symptoms of Connecting Rod Failure

Are you sure he doesn't just have a solid lifter cam and lifters. My 62 340hp sounds like a bunch of marbles rattling around at idle. I kinda like it..
Good luck

[Loren L.]

Re: Symptoms of Connecting Rod Failure

A neighbor once lost the spring on the fuel pump that created a truly expensive sounding noise - isolated the location, removed the pump and $40 later, a VERY happy fella.

[Edward J.]

Re: Symptoms of Connecting Rod Failure

A neighbor once lost the spring on the fuel pump that created a truly expensive sounding noise - isolated the location, removed the pump and $40 later, a VERY happy fella.

Loreen, I have seen a few of the fuel pump springs break on the fuel pumps over the years, it is rare, When they do break the sound almost like a lifter that out of adjustment or a bad lifter. connecting rod bearing failures are alittle more of a heavy knock.

[Duke W.]

Re: Symptoms of Connecting Rod Failure

Connecting rod failures are usually sudden and catastrophic. The only way to tell if you have a fatigue crack, which will ultimately result in such a failure is to remove the rods and have them Magnaflux inspected.

A worn bearing or crank journal will usually cause a "knocking" sound that increases with load. Cold piston slap can cause a similar noise, but it usually goes away when the engine is warm.

Spun bearings are usually caused by oil starvation, and when the bearing seizes, a huge bending load is placed on the rod, which can snap it in two along the beam about one-third of the way up from the big end.

If you suspect a bearing problem, you can inspect them without removing the engine. Remove the oil pan and each rod cap (and main bearing car except the rear) one at a time to inspect the bearing/journal or obvious visual damage.

The sound you describe could also be something simple like a broken sheet metal part - like the front bell housing lower cover or windage tray that is rattling against something.

Duke

[Stuart F.]

Re: Symptoms of Connecting Rod Failure

Joel;

Can you perhaps isolate the source of the noise with a stethoscope? I have had several broken fuel pump springs myself, the first of which I found that way. The sound from the broken cranks all eminated from the clutch housing area leading us to believe it was something in the clutch - again on the first one. After the first one we knew what we were dealing with.

Stu Fox

[Clem Z.]

Re: Symptoms of Connecting Rod Failure

to check the fuel pump just unbolt it from the block leaving the lines attached and start the engine. my new 70 454 450HP chevelle had a engine noise and that is how i found it was the fuel pump on a brand new car.