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JED
 
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Join Date: Jul 2009
Location: Hawaiian Islands/SV Idaho
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Rod bearing issue after top end refresh 3.0

Hi Gentleman-
I believe I may have a rod bearing issue after a my top end re-fresh. My friend Franz and I re-worked the top end on my 80SC while he was vacationing on Kauai, he is a 30 year veteran Austrian 911 air cooled mechanic/shop owner.
At the time we started we didn't have time nor the indications, 118 K, that the bottom end needed attending to. During the process he stated that the case had been split at some point, well a month after he left I noticed a mechanical noise that seems to be deep in the case, also have found a thick silvery sludge on the drain plug. The sound doe not seem to be timing chain related. It is burning a much more oil than in the past and today I noticed a number of small air bubbles on the dip stick. The sound has remained constant but doesn't give me a good feeling. Like a sewing machine and it remains constant at all engine speeds. Seems to be more pronounced at a low idle 750.

I spoke with Franz on the phone and he thinks that when the PO split the case that perhaps they cut corners and re-used the rod bolts, after we added more compression with fresh heads and rings this may have cause the bolts to stretch.

I just wanted to inquire with the experts as to your thoughts. I have heard that I can send the oil for analysis to determine if it contains rod bearing material. At the end of the day I guess I know the answer, I was just hoping to get a few more opinions before diving into the fix on my own. Thank you for your time and knowledge.

Regards-

JS


Last edited by ARCSinAK; 04-15-2015 at 03:58 PM.. Reason: Typos
Old 04-15-2015, 03:55 PM
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More information would help. What type of cylinders are they,(allusil or Nikasill) and how were they prepared for seating the rings? How many miles since the top overhaul? I'm surprised that Franz didn't pull the rods to check the rod bearings and replace the rod bolts when it was all apart. Unfortunately you may have to do this at this point.
Old 04-15-2015, 04:25 PM
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JED
 
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Nikasill we left them alone,

Due to the mileage and time constraints we left the bottom end un touched
Refreshed the 915 at the same time 1 gear sincro's

1500 miles since top end over a year +
Old 04-15-2015, 04:36 PM
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A rod knock will vary depending on rpm and load.

I would listen with a stethoscope to various locations on the bottom of the case when the motor is running. You may be able to isolate the location.

An oil analysis sounds like a good idea.
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Old 04-15-2015, 05:09 PM
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I'm not certain that I would jump to the conclusion that your noise is a bottom end issue. You need to begin by first looking at everything that you touched while doing the upper end.

Start by telling us what your definition is of an "upper end refresh". Was that just a valve job? Or, did you also replace piston rings?

If the engine truly is only 115k miles "old", you should be at about "half life" on the bottom end of the motor. So, I would first look at the work actually performed to find if there isn't a culprit there.

You mentioned that you are now seeing metal shavings in the oil. To me, that is an indication that you are looking at a teardown regardless wherein you will retrace the steps that you took while doing your upper end.

The key to a decision to split the case is examining the rod bearings. If the crank has been damaged due to the failure of a rod bearing, then I believe you should strongly consider splitting the case. In all reality, with the upper end of the motor stripped down to the point needed for head work, you are only about 10-15 hours of labor away from splitting the case and reassembling it.

Just my 2 cents.
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Old 04-15-2015, 05:35 PM
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Value job and rings, all seals.
I understand that most 3.0 bottom ends are bullet proof and normally go for 200k or more when taken care of.
We discussed splitting the case at the time, and in retrospect in would have been a good idea, how ever this issue was not present before the top end work and my good friend Franz went well above and beyond the call of duty on his vacation. We knew the time involved, I trust this mans knowledge completely. I would bet he could assemble a flat six blindfolded.
What I have seen in the oil is more like a silver paste not shavings or slivers.
Franz could tell someone had opened the case in the past, I was just wondering if the theory of the PO re-using rod bolts could be the issue with the added compression of the new top end.
Anyone have a recommendation of a company that will analyze oil to determine if bearing material is present?
The sound is present at all RPMS but is easier to hear at idle. Seems to be worse when the motor is hot.
Old 04-15-2015, 10:47 PM
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You are running blind here really without a another tear down,little you can do except as already stated get an oil sample done,the sample will pick on the white metal if the big end bearing are on the way out,the sample may also pick up other metals but will show the main cause you suspect....if thats what it is.
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Old 04-15-2015, 11:41 PM
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Does the knock vary with load? When I have heard knocking rod bearings, they would change intensity as you accelerated and decelerated the engine. IIRC they are loudest at coast. Piston pin bushings are similar but more of a rapping noise than a knock. If the knocking is just constant it might not be bearings, but as we all know there are many other things that can make bad noises. If you haven't tried a stethoscope, you might see if you can pinpoint the location.
Old 04-16-2015, 04:54 AM
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Harbor Freight has a reasonably priced stethoscope, worth the investment, IMO.

I would certainly pull the valve covers and do a careful inspection of all the rocker components. A loose rocker shaft can make a lot of noise.

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Old 04-16-2015, 08:13 AM
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