New (used) lower unit, a new noise, an other variables

[Wood Zeppelin]

I was able to find a used version of the upgraded lower unit design for my 1998 Honda BF40. I got it all put together and went for a test drive today. There is a noise I never heard before, kind of a "whirring/ringing" sound that occurs between about 2000-2700 RPM.

With the motor in neutral, I can manually spin the prop just fine, no noise or anything. Here are the things that have changed since before the noise:

New (used 2007) lower unit.
New impeller
New gear case oil
New prop (different pitch as I've gone from sea level to 3000' elevation)
Elevation change: using the 3000' prop, but I'm now running at 6000' elev.
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My fear is that after dropping $1000 on a *used* lower unit, there's something wrong with it?

Or could it be something else?

Should I even worry since it sounds just fine above and below that RPM range?

[Salmon Fisher]

I would put the old prop on and see what happens.

[T.R. Bauer]

It is a gearcase and sometimes they make noises. Why? The gear contact pattern was not set up correctly, or they are worn and that is basically about it. If the noise is subtle, and the tolerances are ok inside the case, it probably is just annoying and nothing else will come of it. But the prop can cause strange noises too.

[T.R. Bauer]

If you run for a few hours and then drain the lower unit oil and there are metal shavings, you have issues. Can you take it back?

[Wood Zeppelin]

[Wood Zeppelin]

[T.R. Bauer]

[ATPNW]

Did you get all the thrust nut/washers back on and in the correct place? Sometimes they stick to the back of the prop when you take them off. If you are missing one they can definitely make that kind of sound. You can easily tell because the back of the prop housing would be grinding metal on metal to the gear case housing. Depending on how long it was run, there might also be some free play forward and aft of the prop

[Wood Zeppelin]

[Wood Zeppelin]

[T.R. Bauer]

The prop will not impact the wear pattern on the gear set because the end play and bearing tolerances are not going to move regardless of what prop you put on it. To a small degree they are dynamic, but they are as close to static as you can get. If they were not, then you would have serious problems.

And yeah, like another person said, that thrust washer thing happens to people. But in my experience the result is catastrophic and nearly immediate. And something that will require a new aluminum case because it doesn't take much to wreck them. There might be exceptions and somebody may have gotten lucky - I just don't know of any.

The only thing I can think of is that the sea level prop changes the rpm and moves the wavelength of the soundwave to a place in the spectrum that you can't hear it....meaning it's still doing it and you just can no longer hear it. If you think everything is correct with your installation, it would not be the first gearset that makes a bit of noise with nothing really wrong with it. It's really pretty common.

But I'm glad it's gone. And I hope it stays gone. When you drain your gear oil in it at the end of the year and there are lots of pretty sparkles in it, you have real issues. If the prop shaft starts leaking it is indicator something inside isn't right as well. Hopefully none of that happens.

[Salmon Fisher]

There's also a thing called "prop sing."

Google it and you'll learn a bunch. Might be what you have with that noisy prop.

On a another boat I had, changed to a different prop and it made noise at a certain rpm. No noise under and above that rpm range.

[T.R. Bauer]

I didn't think prop "sing" was an issue with outboard motors. I learned something today.

[Wood Zeppelin]

[Wood Zeppelin]