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Old 25-03-2007, 04:45   #1
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Yamaha 23hp Diesel Engine - bearing replacement

We have a rare 1983 vintage YAMAHA 23hp diesel engine. It needs some surgery but is still in the vessel.

Is it possible to replace the big end bearing on the con rod without pulling the motor apart, that is can it be accessed from the sump ?

Anyone got any experience with this motor ?
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Old 25-03-2007, 13:47   #2
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Normaly, if you need to do a big end bearing, you have other issues as well. Just the big end alone, You will need a shaft grind and oversize bearing shells to then suit.
But normaly you will need to do a rebuild. If the big end is worn, then more than likely the engine is worn and needs a rebuild.
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Old 25-03-2007, 18:41   #3
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The front big end is the only bad symptom, it goes BANG BANG BANG BANG etc, only at medium to high revs does it reduce the noise level. I have service the injectors, so that is not the cause. The motor is set at an angle of 18-20 degrees and that may have been a contributing factor, as well as years of neglect before we rescued the vessel.

If we do repair the motor it is probably not a long term solution, as we need to address the shaft angle issue (rebuild the stern or get an gearbox with an angled output). Hence we want to economise on the parts as they are very difficult to obtain and most need to be brought in from Japan by the supplier, the motor is so redundant. They said some parts could be 3 to 6 mths to obtain and then some may not be available at all.

Now if it was a Yanmar it may be a different story, but it is a Yamaha.
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Old 25-03-2007, 23:21   #4
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Hmmmm, I still don't think this will solve the issue. If the wear was caused by poor lubrication of that bearing due to motor angle, then the shaft will be badly worn as well. If it goes bang bang, then you most likely have a very out of round worn shaft. Also if there are wear marks in the shaft, a new bearing will quickly be torn apart. The old bearing will have the same opposing marks.
However, leaving all that aside, removal of the sump should allow you to get at the big end. You will most likely not be able to remove the piston, but you shouldn't need to anyway. Replace the bearing and see how it goes. But my thoughts are that it will fail not long after.
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Old 26-03-2007, 17:02   #5
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Hi Alan

Thanks for the advice. I agree that it is not ideal, if it helps at all it will be just a stop gap until we can procure a better motor. The top end seems OK as it starts straight away, produces good power for it's size and does not blow smoke.

The motor is installed with the sump an inch above the crossmember so the motor will have to be removed from the vessel to look inside.
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Old 27-03-2007, 00:59   #6
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Are you absolutely sure it is the big end?? I am not sure what sort of Bang you "bang" is making. You would hear a harsh knocking sound from the internals. But then it is quite possible it is something else knocking internaly as well. Like maybe even a broken bolt on the big end for instance. I would suggest a look at it ASAP without running the engine anylonger would be a good idea.
I found that out the hard way once. I had an engine with a bad clatery knock. I gave the engine a quick rev to hear the noise vary and try and get a hold on what it maybe. But unfortunatly the knock suddenly turned into a loud banging noise as the connecting rod flew out through the block and blew smoke and oil all over the place.
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