Rod bearing lost spread

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Retroboy

Retroboy

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What causes rod bearing to loose their spread. Crank was ground, Rods resized and measurements checked. Engine has done 12 passes on 1/8 mile. Some evidence of oil starvation on the top half but I put that down to a little bit of fuel dilution (alcohol fuel).
 
They can be taking a pounding above & beyond the normal combustion process. That would contribute to the extra pressure & wear on the top bearing half. Overheating can also cause them to lose their "fit". Pics might help someone interpret what's going on.
 
I was just looking at the head that's off and the exhaust is showing some heat. I had purchased an EGT gauge but not fitted yet because I suspected it may have been a little lean. I chuck a new set of bearings in and see how it goes.
Cheers
 
Retroboy said:
I was just looking at the head that's off and the exhaust is showing some heat. I had purchased an EGT gauge but not fitted yet because I suspected it may have been a little lean. I chuck a new set of bearings in and see how it goes.
Cheers
Click to expand...
 
Okay brains trust. No trace if detonation on the plstons, checked the rods for round and size. Still looking for answers?
 
AL or tri-metal bearings?

What is telling you that the spread is going away? Just not a tight re-fit into the rod ends? Do new bearings have the proper tightness of fit? Or... ??
 
If the rods were reconditioned have the big ends rechecked. They may not be right. Were the oil clearances checked and correct before assembly. Bearings can loose their crush but it's usually caused by incorrect machining.
 
Al bearings, new set fit the rods no probs, no evidence of detonation, less than .0005" difference between rod sizes. .0025" bearing clearance when assembled. The now bearings fall out of the rods. As I said some signs if heat in the combustion chamber suggesting to me it's little lean. NGK Bp6es plugs look good nice as far as heat on the earth strap. This is my first Chrysler (360 LA) and may be the last. It's been a heart breaker from the get go. I don't want to just put it together until I can find a cause.
Cheers
 
Just for you to consider....

Here is a paper from King Bearings describing the different structures and materials used in their bearings. It can be seen in the comparative table in section 5 of the paper that all of the AL bearings have lower fatigue strength than any of the tri-metal types, and the softest AL ones are considerably lower.

http://kingbearings.com/files/Engine_Bearing_Materials.pdf

Just running some quick numbers, it is not hard reach peak rod forces well over 5,000 lbs with a 4 inch bore, and so it will be easy to reach some pretty high psi numbers on the rod bearing surfaces. And, with those 12 1/8 miles passes alone, you have put over 5,000 heavy load cycles on each bearing. So, based on the King data, a common passenger car AL bearing would seem suspect.
 
With alcohol in the oil the bearing will get warm and shrink to the crankshaft
 
Just because you can't see anything doesn't mean you ain't into detonation.

I don't use the earth strap as a measure of spark plug heat range. Look at the shell.
 
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