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  #1  
Old 04-07-2005, 10:30 PM
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Engine disassembled but can't find out why its knocking, Mr. Leathermang





I have my engine apart. It had a bad knock due to oil loss to .5 quarts when I accidentally plugged my breather hose.

All the connecting rod bearings are scored some badly, some not too badly. Everything else is okay. As bad as the knock was, i thought I would literally find a badly galled bearing but i didnt.
I will post bearing pictures in 1 hour. What should I be looking for? The connecting rods seem okay.
Could worn connecting rod bearings alone caue this knocking?
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  #2  
Old 04-07-2005, 10:46 PM
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.5 quarts or 5 quarts low? If the rod bearings were bad enough, you could get a knock. The main bearings are okay? Have you measured the clearance in any of the bearings? If not, you need to use plastigage or some other method of ascertaining all bearing clearances and post the results. Clearance is the most important aspect of journal bearing condition.

Good luck.
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  #3  
Old 04-07-2005, 10:50 PM
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By connecting rod bearings.. do you mean rod to crank only... or rod to piston pin also ?
Because of the way in which you can open up the engine you get to see the crank and crank/rod bearings first... so if you find something obvious which can be fixed there you have the option of leaving the pistons and rings alone...
But now it looks like from your description that you will need to go further and check the piston to pin fit and the connecting rod to piston pin fit...
These are the smaller surface area and higher load positions in the stopping and starting the piston the other direction and the first place the combustion pressures impact a bearing surface... 2000-4000 TIMES PER MINUTE... a lot of action happening there... that is why tolerances are so much more important on a diesel than a gasser... 8-10 to one compression ratio compared to 21 to one...
This is the point realistically that you should consider that other used engine you had a lead on..... from what you describe this will be a total teardown if you decide to continue.. which means finding competetant Crank working people... not easy.. and surely very expensive... and this is without the potential piston costs..
Consider bailing on this engine and turning it into that winter project you talked about...
He ordered a bunch of plastigage... so I am sure those measurements will be forth coming.. although much of what I said about the crank still stands...
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  #4  
Old 04-07-2005, 11:23 PM
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I remember I was watching a video of a 1k hp Supra that blew its engine at 8k rpm at 200mph. It sounded just like a diesel pulling into the guys driveway after the run. They showed pics of what went wrong, 5 of the rod bearings looked fine, but number 6 was scratched up pretty baddly and the crankshaft was to.
It doesn't take much, I would send the crank out and replace them all.
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  #5  
Old 04-08-2005, 12:25 AM
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In the other thread I think he said he had a good engine lined up for $500 .. so given the findings I still think he will get by way cheaper to get that one now....
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  #6  
Old 04-08-2005, 12:48 PM
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Carra,

Piston rings can also be dmamaged by running dry of oil. Only way to tell is a compresssion test. How do I know: Happened to me Replaced bearings after oil pump failed but compression was so low, the engine wouldn't start cold.

Bad rod bearings will make an engine knock. What's important is the condition of the crankshaft. If the bearing journals are scored, the crank must be reground.

P E H
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  #7  
Old 04-08-2005, 01:17 PM
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Engine assessment

The engine actually has very good compression so I don't want to touch the piston's in their bores. However it knocks badly on a cold start and then knocks moderately.
After the oil loss the engine knocked so badly for a week I thought it would blow up any second or any minute or anyday. But it turned out the oil loss had damaged the fuel pump and when I replaced it with a spare pump, a lot of the knock went away

The crankshaft journals look okay. The connecting rod bearings are all scuffed.
The crankshaft main bearings also look okay.
Because the connecting rod's dont get as much lubrication in normal operation as the the bearings I would think they survived since the piston rings survived. Also if the knocking was from a bad connecting rod, perhaps the rods should have failed by now.

Its a shame the engine looks clean inside. I was thinking of replacing the connecting rod bearings and then firing the engine up on a test stand with radiator and exhaust hooked up in my garage to hear what it sounded like before putting it in my car. I did that before when I rebuilt engines, dont laugh.

The thing that makes me wonder is it knocked horrificly once and I expected to find something really amiss when I opened the engine.
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  #8  
Old 04-08-2005, 01:42 PM
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like Greg suggested, check the piston pins and adjacent bearings at the top of the connecting rod. I know you don't want to pull the pistons, but you might have to. One possible alternative - mount a dial gage so that it rests on the flat bottomed surface of the con rod(s), and try to pull the con rod up and down to see if there is any play. Maybe you might be able to flush the journal with rubbing alcohol and compressed air to remove the oil film so you can measure the play. You shouldn't be able to measure any play unless you have one heck of a gage and technique to match. The fit is something like 10 microns IIRC?? Anyway, if the rod bearings are scored due to insufficient oil, it's a pretty sure bet that there was low oil to the rod to pin and pin to piston fits. Plus, the width of your ring grooves could have opened up. This is a very cool engine to rebuild, but, a good used engine is sure a nice option.
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  #9  
Old 04-08-2005, 02:14 PM
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Here is a thought ... as what Pete has said is right on the money....
How much distance is there between the bottom of the lowest ring groove and the top of the piston pin ?
I wonder if careful working could bring it low enough to replace the piston pin without disturbing the rings in the bore ?
A very long shot all things considered... but I know you don't want to go to the next step dealing with the bore...
This is a little more feasible given your access to a backup engine.... and your experience rebuilding engines in the past...
It is certainly a gamble given what has been found to replace the bearings which you can access and fire it up.... your assumption about the lubrication to the rods feels more like wishful thinking... but the movement is much less than at the crank/rod connection... so you can always hope...
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  #10  
Old 04-08-2005, 02:36 PM
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Once those pistons come out of the bores, it turns in to a total rebuild

Because I have to pull the head to stick them back in and once i do that, I would send the head out. Then they say you have to put in new sleeves and pistons. Except for the money I am dying to do this. Its great a feeling having an engine apart and to rebuild it.(Maybe all that degreaser is giving my brain a twist!)
I may not be a pro (I've done 10 rebuilds only) but usually I keep on asking questions and at least try to get it right on paper..but I love doing it except diesels are very expensive to rebuild.

My question is "How true is that you have to resleeve and repiston a block?" On my 240D diesel I didnt resleeve and it burned a lot of oil a year later (maybe it was cheap Deves rings..) on my Volvo Diesel i did not resleeve but it did not burn oil
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  #11  
Old 04-08-2005, 03:52 PM
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I wonder if the vacuum pump could cause the same noise. I've heard they can cause very similar noises when they've gone bad.
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  #12  
Old 05-23-2008, 02:09 AM
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carrameow, what happened with this rebuild? I am in a similar situation?
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  #13  
Old 05-23-2008, 02:15 AM
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measure crankshaft with a mic. measure the conrod's (and check for straightness) and measure the main caps (the diameter) check for piston slap.
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  #14  
Old 05-23-2008, 10:39 AM
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If there is ANY scoring on the crank or connecting rods, you must replace or regrind the crank and the rods. Then use over size bearings.

Best and most expensive option is a rebuild.
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  #15  
Old 05-23-2008, 11:10 AM
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rgdassi,

To do the things the things Cervan suggests, U will have to take the rods to a machine shop. I doubt if U have the proper tools to do this yourself.

Measure the crank journals at different places to determine if any are out of round.

P E H
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