Connecting rod failure analysis?

[Macrobb]

I've just torn into the motor in my tan '88, the one that blew 18 months ago.
As I knew, it was a connecting rod that broke. The interesting thing is that the bearings on both ends were fine...

The connecting rod broke perhaps 1/2" from the 'small end' in the piston, sheared at approximately a 90 degree angle to the rod(square cut).

The bottom side, well, I can't tell much at this point, because it's twisted and gnarled from punching several holes through the firewall, block etc.

Now, even broken, the lower end rotated smoothly over the crank. Pulling the rod cap off, the bearing looked fine - I saw small scratches lengthwise(around the circumference) on it, but nothing I could catch with a fingernail... so pretty small.
The upper end was jammed in place - the sheared end managed to jam itself into the side of the piston skirt. Once I pulled the pin out(hammer and screwdriver), I found that the upper end slid smoothly onto the (now removed) pin, and again... no wear, no massive clearance...

The top of the piston was in good shape; covered with an even layer of black soot, just like all of the other 7. The only obvious(from the top side) sign of failure was the piston being rotated about 15 degrees from where it should have been.

I also saw no other issue with any of the other 7 rods; everything was smooth and tight. Nothing bent.

Any ideas on why this would break in this manner?

I found one reference to breaks like this being due to overrevving, where too much tension happens against that rod and it finally snaps. This would be possible - 6-9 months before the failure, I had the engine revved up to 4500RPM several times, before I finally got fed up and 'fixed' my RD2-110 IP to have a reasonable governor.
Because of that fixed governor, I hadn't revved it too high for months before it finally failed...

I'd like to post pics, but don't have them today.

 

[Thewespaul]

I’d have to see it to give you a certain answer but my gut says that rpm has something to do with it. That combined with it being a 30 year old casting you just probably found the weak link when pushing it, being able to see how it broke should be able to confirm that. Was this engine originally timed 180 out or was that another truck I’m thinking of?

 

[Macrobb]

I’d have to see it to give you a certain answer but my gut says that rpm has something to do with it. That combined with it being a 30 year old casting you just probably found the weak link when pushing it, being able to see how it broke should be able to confirm that. Was this engine originally timed 180 out or was that another truck I’m thinking of?

This motor was professionally rebuilt, screwed up(Passenger's side head was *not* to spec, resulting in piston to valve contact. Mainly #1 exhaust valve), and run for a total of about a year, less than 10,000 miles. Yes, the connecting rods were original. What's odd is that I would have expected #1 to fail, not what failed(#3).

Nothing 180deg off that I know of; it always ran OK. It really didn't like decompression gaskets, though, for the month or so I had them on it(about 6 months after it was installed, 6 months to failure - this was after I found the chuffing due to the valve guide being... gone; it did get a new(used) head at that point).

 

[Thewespaul]

Possibly hydro locked that one cylinder with the decompression headgaskets?

 

[DrCharles]

I found one reference to breaks like this being due to overrevving, where too much tension happens against that rod and it finally snaps. This would be possible - 6-9 months before the failure, I had the engine revved up to 4500RPM several times,

We have a winner... I think you overstressed the rod in tension a few too many times. Many high-rpm rod failures are actually from the bolts stretching then breaking, or a spun bearing from insufficient clamping force, not the rod itself. But as you pointed out, the big end still slid on the crank journal and the bearing looked ok.

 

[Macrobb]

Here's some pictures:

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[FarmerFrank]

"Fatigue"


Sent from my iPhone using Tapatalk

 

[david85]

So here's my question: Is that bigger wrist pin on the turbo IDI really worth the extra weight? Forged rods sound nice, but it seems to me the wrist pins are plenty strong as they are.

 

[genscripter]

We have a winner... I think you overstressed the rod in tension a few too many times. Many high-rpm rod failures are actually from the bolts stretching then breaking, or a spun bearing from insufficient clamping force, not the rod itself. But as you pointed out, the big end still slid on the crank journal and the bearing looked ok.

From a mechanical dynamics engineering standpoint, the rod cap studs should have failed from tension or thread sheering long before the rod could have snapped. Cross-sectional area is substantially larger than the rod cap studs and nuts, but then again, the studs are a higher grade steel. Even with the steel grades accounted for, the rod itself should have outlived the the threads or tension resistance on the cap studs.

That's why this cap, in my estimation, failed due to a metal fatigue over a 30-year lifespan of vibration and revolution. This is also why you didn't find any other major failures or wear patterns on the crank or rod bearings, because this was a one-off failure specific to the metal fatigue of that particular rod.

 

[Macrobb]

Is there anything I can look for when assembling a motor to try to prevent this in the future?

So here's my question: Is that bigger wrist pin on the turbo IDI really worth the extra weight? Forged rods sound nice, but it seems to me the wrist pins are plenty strong as they are.

I don't think the added wrist pin diameter matters. The extra added cross-section should help though.
Also, pretty sure these N/A rods are forged as well...

I'M interested in an added bushing for PSD rods so we can use stock N/A pistons.

 

[FordGuy100]

The inertial component is large but 4500 RPM doesn't have too high of a mean piston speed.

 

[Thewespaul]

That’s a clean break. The material failed there, and with the rest of the rods being fine I still think it’s just the 30 year old casting that failed you, manufacturing wasn’t perfect in 1988 and it still isn’t, things like this happen. Nothin you can really do to prevent it happening in the future other than the typical inspections you should already do during a rebuild, it’s just apart of working with old iron.

 

[david85]

Could magnaflux detect early stage fractures?

 

[saburai]

Could magnaflux detect early stage fractures?

I'm not sure, but it couldn't hurt.

Don't have anything to add but thanks for all of you who are pushing the limits on our old iron! Threads like this are full of excellent information.

 

[FordGuy100]

Can we get side views of the connecting rod where it broke? It'll be easier to see if there was necking or if it was a brittle fracture.