Johnson 40 1992 lower unit knock

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cjr1971

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Hello All.
I am new to this forum so hello to all.
The reason for posting on this forum is I need help.
Yesterday I took my boat out to go fishing. She started with no drama. I took her out of the bay and got up on the plane, again no problems.
After about 2 miles there was a knocking noise. fearing the worse I stopped the boat. The knocking had gone. I revved the motor and she was sounding very nice. I put her into forward and there was the knock not loud but noticable. the knock sounded like it was coming from the prop.
To cut a long story short. I idled her back to shore and got her home. Once home I dropped the LU oil to check. The oil which I changed in march was charcole grey. On the LU oil plug there was quite alot of metal filings with small chunks of metal mixed in for good measures.
Now my question is. what does all this mean? Do I get it fixed or buy another LU.
If anybody has any advice Please post. Thanks in advance for all responces.
 
fireman57

fireman57

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Messages
3,811
Re: Johnson 40 1992 lower unit knock

It needs reubuilt because the water is not a good lubricant. Rebuild will probably cost around 1200.00. If you can find one already rebuilt cheaper and it is a reputable place I would put it on myself.
 
C

cjr1971

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Re: Johnson 40 1992 lower unit knock

Thank you for your reply Fireman57
When I dropped the oil there was no water in the case or any milky substance in the oil.
 
fireman57

fireman57

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Re: Johnson 40 1992 lower unit knock

you said it was charcoal grey. It will be golden if it was fresh oil, grey had water in it. You also might have had a worn gear that gave way when you pushed into fwd/rev when your engine wasn't at idle. It also might have been that it's number was just up. If you have metal shavings in there it is time for a rebuild. If I remember correctly the gear set is around 650.00. That doesn't include labor.
 
5150abf

5150abf

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Re: Johnson 40 1992 lower unit knock

Concur, shavings in the lower is really bad, either somehting let go or is about to, you did well to bring her in, alot of guys woudl have run it till it broke.

The reason metal shavings are so bad is.... if the engine is running the gears in the lower are turning, the drive shaft comes down from the power head and ends with a pinion gear, the forward and reverse gears are on either side of the pinion, between the forward/reverse on the prop shaft is a clutch dog that slides back and forth and is what selects the gears so the metal shavings have been roilling around in the lower getting crunched up by the gears.

I have gotten really good deals on lowers on Ebay, way cheaper than having them rebuilt or even just buying the gears but either way you need a new lower.
 
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Haffiman

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Dec 17, 2009
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2,454
Re: Johnson 40 1992 lower unit knock

Drop the lower unit, remove the water pump and upper bearing housing.
The bearing had a history of failing there!
That even resulted in the shaft wobbling and the seal could not handle it.
 
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cjr1971

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Re: Johnson 40 1992 lower unit knock

Thank you for all the replies.
I don't think I will mess about stripping the LU. I took all your advice and looked on ebay. Then I was Very nice to my wife. HAY PRESTO bought a replacement LU for a good price.
Is there anything I should look for when I get the LU.
 
H

Haffiman

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Re: Johnson 40 1992 lower unit knock

Change the oil and install with new impeller.
 
fireman57

fireman57

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Re: Johnson 40 1992 lower unit knock

Ditto. Hope that it is sealed properly also.
 
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cjr1971

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Sep 17, 2009
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Re: Johnson 40 1992 lower unit knock

:redface: not that one. But one for a 1992 johnson. Is it worth me resealing the gearbox. Is there any tell tale signs that the replacement LU is bad.
eg. metal filings in the oil, milky oil etc?
 
H

Haffiman

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Re: Johnson 40 1992 lower unit knock

If not mistaken it was an old Service Bulletin issued concerning those lower units and the upper pinion thrust bearing, probably the one that failed on yours.
Given the 'same old' unit, I would have ripped apart and checked the whole unit AND resealed.
 
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cjr1971

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Re: Johnson 40 1992 lower unit knock

I just pulled the LU. It's in great shape. I might just rip the LU apart and rebuild. Are the bearings you mention just under the wear plate?
There is a little movement in the drive shaft maybe 0.5mm But the bearings look pretty good.
I know if I want to keep the LU I will have to strip though.
Pity really considering everything else is in great shape.

I had a Suzuki with LU Probs before this engine. The LU seized up on me. When I stripped the LU it did not look that complicated.

Do you think I could rebuild myself?
 
D

Daviet

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Sep 24, 2008
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8,958
Re: Johnson 40 1992 lower unit knock

Get the factory manual and read through the lower unit section a couple of times and see if you think you can do it. With the factory manual and the help of the people here I would think you could do it.
 
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cjr1971

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Sep 17, 2009
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Re: Johnson 40 1992 lower unit knock

I took the prop off and found the culprit.
Fishing line had found its way down the prop shaft and got into the bearings. There's my leak.
I looked at no. 38 . . . I'm not sure if it is the old or the new I have. I will change it anyway just to be safe.
I don't want to give up on this LU because it is in A+++++ condition (except for the water and metal in the oil).
I will deffinatly get a manual and give it a go.
In for a penny, In for a pound as they say in England. (Even though I'm Welsh)
You guys have produced a priceless source of information for me, and your all an insperation. Thank you
 
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