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Lower unit noise/Play in prop shaft

J

jn1440

Contributing Member
Hey everyone got a question about the lower unit on my 1984 Evinrude 70HP. I have noticed that there appears to be a knocking noise coming from the lower unit area of the engine underway only. Upon further inspection it looks like there is side to side play in the prop shaft, not very much just enough to feel it but not really see it. No water in the oil and it shifts and holds gear fine. Is this expected from a 30 year old engine or should I expect it to explode at any moment?
 
The shaft is supported in a needle bearing. Needle bearings cannot be run "tight"--they must have a very small amount of clearance. You must be a sensitive person if you detect it. So, it is hard to say. If there are no leaks, and it works ok, and there are no metal grindings or chips in the oil, probably safe to say it is ok.

BUT you say it is making noise. That changes the whole story. If it truly is making noise in the gearcase that you can hear in the boat, I have no idea what it is, or what advice to give. One thing about it, if it really is making such a noise, it probably won't do it for long.
 
+1 to gator, is the noise in accordance with the rpms or is it a constant rhythm? is only when in gear and running?
 
It's been my experience through the years that when a lower unit starts knocking, the cause has always been a chipped tooth, broken tooth, some imperfection in at least one gear... but... normally it affects two gears at least.

When you've changed the gearlube in the past, have you noticed any metal shavings, small chips, steel splinters?
 
That's kind of what I had in mind when I said it won't do it very long. It will get much worse, very quickly when the rest of the gears go to hades.
 
When I changed the oil at the start of the season it was still clean, no chunks no shavings maybe a small amount of water less than a few drops. I also wonder if this noise could be me unable to tell the difference between normal engine noise and a problem. These old two strokes make a lot of noise I think I have thrown a rod at least once a week. The noise however has been present for approximately 10 hours so like said earlier if something was wrong enough to knock it would have died by now. Only other concern would be the prop making some sort of noise or trouble. The current prop is a little dinged up, nothing major but along the leading edges its a little jagged with a few dings here and there nothing larger than a pea though. I have a terrible time keeping props in good shape I would replace at the start of season but would strike the prop that day without fail every year so I have gotten a little behind on prop maintence I hope that's not causing any damage?
 
A highly damaged prop will cause an unbalanced condition,vibrations,thus damaging bearings& seals,losing l/u oil.$$. Small dings can be filed.Have u tried broomstick& ear scope method on lower unit??
 
Not to deny that a highly damaged prop can cause damage, but I seriously doubt that a few dings the size of a pea as described are going to hurt anything.
 
It's hard to tell because it only occurs past 3500 rpm. Which the way my idle lever is set up will only allow 2000 rpm.
 
Ok this one is an update, I pulled the prop off to take and have repaired and when I did I noticed the prop nut was fairly loose. Loose enough that the prop was rocking on the shaft a bit. There is still a small amount of side to side play in that shaft but not as much now that the loose prop is gone. I will get the prop repaired and put back on tight then report back. Unrelated question, is there any resources for choosing the right diameter and pitch for props. This prop is a 14" diameter which seems a little excessive for a 70hp on a 17' Boston Whaler. I don't think its pitched right either its at a 13 pitch but if I go to WOT the engine will rev well past 5500 if I let it.
 
Do not remove shift cables & rev in neutral at full throttle,The engine needs a load on it,prop pushing water! Otherwise damage will follow $$$$.
 
Didn't Rev without it in gear don't worry. But I did get a new prop, same as the old one just less damaged and this new prop is giving me trouble. There is a squeak like a belt squeal at 3,000 to 3,500 and beyond that I'm getting heavy vibrations. I guess it's the new prop but that seems very odd.
 
Change prop with old one u just took off. Any difference? Is this new prop brand new out of the box? Are motor mounts secure on lower unit
 
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