Excessive Rod Bearing Wear

[Boze]

I recently developed what sounded like a rod knock on a rebuilt L134 RMC engine with 500 miles on it. Upon tearing it apart (again), the #1 rod bearing was extremely scored and 2, 3 and 4 were also scored but to a lesser degree. The mains were all perfect. the pistons and cylinder walls are all perfect.

The crank was reground to .040 under during the rebuild and .040 bearings were supplied by the shop. The rod journals were just measured and are all a couple of ten thousandths greater than the tolerance spec (.0008-.0023), not counting the depth of the scoring. This means there was too great of clearance between rod bearings and journals. The scoring also appeared worse on the middle of each upper half. This would correlate to the compression and power strokes of the cycle.

There were excessive carbon deposits on the exhaust valves, piston tops, top rings and cylinder head from combustion chamber to exhaust valve.

The crankshaft oiling shafts were all checked and were clean as a whistle. The oil pump was new at the rebuild. (Melling)

The engine did not run hot, did not seize or lose power and did not blow smoke. There was never less than 4.5 quarts of oil in the engine and the filter elements were new. (Fram)

I am puzzled at the wear of the rod bearings and am considering these possible causes: 1. excessive slop in rod journals caused insufficient lubrication. 2. 10W30 oil used is too thin for summer use. 3. Possible dilution of oil by possible undetected leakage from fuel pump (civilian) and excessively rich carb mixture. I did not smell gasoline in the oil when I changed it earlier this year or now. (I am using a WO carb.) 4. the #1 rod bearing was partially starved for oil by oil filter supply line. (Fram Jr.) 5. Undetected low output from new oil pump. 6. Defective bearings. 7. Combination of above.

My oil pressure gauge was not working recently and like a dummy I failed to correct it right away so I do not have an accurate reading just before tear down. But, there was good pressure last year when I hooked up a temporary gauge to the motor. If the excessive clearance at the rod bearings are the main culprit then the pressure could have been fine anyway, right? And oil was being pumped up to the filter.

Has anyone encountered these symptoms or have an idea as to the cause of the bearing wear? I don't want to repeat any mistakes made and tear down the engine a third time. I have secured a new crank and bearings. The engine will be thoroughly cleaned (again) before reassembly and the carb adjusted. I will use 30W oil or 20W50. Is there a way to test the output of an oil pump without running the engine? It appears to be OK.

Thanks in advance for any help.

[Brian]

I have seen a similar problem when the rebuilder did not reface the connecting rod caps to the connecting rod and re-bore the rod. I witnessed two engine failures due to this problem. Apparently the rod caps will become out of round over time and on the rebuild with new bearings there is too much pressure on the bearing surface to hold up.

The first motor made it a little over 200 mi before it started knocking. The oil was real shiny with all of the bearing material in it.

-Brian
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Brian '51 M38 USMC

[oilleaker1]

Rods should be re-sized correctly, were they in backwards with the offset incorrect? Pre-lube when starting, a must. Plastic gauge at least one of the rods and mains upon assembly. Oil pressure is very low with too much clearance. Std. oil filter or add on spin type filter? Any plugs out of the gallery including the oil sprayer up front? Lots of threads on re-build with pictures . John