Knocking sound coming from outdrive

KevlarMT

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May 24, 2011
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I have a 1987 boat with an OMC outdrive. I took it out for the first run earlier this year and noticed a knocking sound coming from the outdrive. The knocking sound is not present or is very quiet when the boat is not in gear. When the boat is in forward there is a rhythmic knocking that increases as the throttle is increased. It seems to get louder when the outdrive is turned right or left. I don't notice the knocking in neutral or reverse. I checked the oil level in the drive and the adjustment of my shift cable and took it out again yesterday. The knocking is still there. I pulled the outdrive this morning and checked the gimbal bearing and u-joints. There is no sign of water in the upper bellows, the gimbal bearing moves smoothly and has no indication of water damage or wear and the u-joints look clean and almost new. I took the upper cover off the outdrive and inspected the gearset in the upper portion of the drive. The oil is clean and the gears look very good. I couldn't see any signs of wear or damage. What am I missing?

Thanks for your help!
 

KevlarMT

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May 24, 2011
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I haven't checked that yet. I still need to drain the rest of the oil. I only removed enough to see the upper gearset. I know it's hard to locate noises when the engine is running and on the water but it sounds like the noise is high in the drive. I'll drain the rest and look at that magnet.
 

KevlarMT

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I drained the outdrive and removed the lower drain plug. I don't even want to type the rest of this message - the magnet is covered in metal filings and there was metal in the gear oil as it came out. The upper gearset looks great so it must be the lower drive that is toast. What do I do? Also, how does something like this happen so I can prevent it once I get things fixed?
 

bruceb58

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Mar 5, 2006
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30,454
The knocking is likely the dog clutch releasing and then catching. Prevention is adjusting your lower shift cable properly with the proper tools.
 

KevlarMT

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May 24, 2011
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Is it better to try and replace the lower gear set or look at buying a new lower unit? Here's a pic of the gear set in the upper unit.

Upper Gear Set.jpg
 

bruceb58

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You aren't going to find a new lower unit if that is what you are asking. If you are talking used, yes, you can find those.
 

KevlarMT

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May 24, 2011
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I suppose what I am asking is whether I should attempt to rebuild my lower unit or if it is more cost/time effective to purchase a rebuilt unit. Here is one I found that could work: http://www.sterndrive.cc/product/Fits-OMC-Sterndrive-OMC-Cobra-0985586.html It appears this might also be available to order through iBoats.com as well. I do most of my own automotive repair work. Is a rebuild something that would be worth it for me to attempt?
 

bruceb58

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I would find something used or go the SEI route You will spend a lot in special tools if you rebuild yourself.

Problem is, you have a 1987 boat. How much is the boat worth?
 

KevlarMT

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May 24, 2011
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It's a Mirage Intruder 22' Cuddy Cabin. I think it is probably worth around $5000 - once this outdrive problem is solved.
 
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KevlarMT

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May 24, 2011
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As an update to this question: The problem turned out to be 2 broken teeth on the forward gear of the lower gear set. Everything else was in great condition. The repair guy said that the lower unit had been shimmed incorrectly when it was first installed (about 11 thousandths too far) and that may have been what broke the teeth. I have a new gear set in it and have installed it on my boat. I just need to check, double check, and triple check my cable length and adjustments and then I'll get it on the water and see how things work.
 

KevlarMT

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May 24, 2011
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Yes, I have both the tools needed. I've done enough research to discover that there is one right way to do it and many, many wrong ways.
 
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