Lower unit grinding noise

[gobucs1]

I have a 1984 Johnson 25 HP that I recently blew the lower unit. About a month ago I redid the waterpump and corresponding seals. When the lower unit was off I could turn the driveshaft and it would make no noise. This is my reason why I believe the noise I have now should not be present.

I bought a 1984 Johnson lower unit today to replace mine. This lower unit makes a noise when you turn the driveshaft that sounds like something is grinding. I took it apart and the gears looked like new and nothing looked out of place. When reassembled and re-oiled the noise is still present. I do not want to put on motor and run and have this lower unit fail if this noise is bad.

Both units were off motor when above symptoms occur. Old lower unit did not make any noise so that is why this lower unit concerns me. Any suggestions to try?

Thanks!

 

[F_R]

Re: Lower unit grinding noise

I have a 1984 Johnson 25 HP that I recently blew the lower unit. About a month ago I redid the waterpump and corresponding seals. When the lower unit was off I could turn the driveshaft and it would make no noise. This is my reason why I believe the noise I have now should not be present.

I bought a 1984 Johnson lower unit today to replace mine. This lower unit makes a noise when you turn the driveshaft that sounds like something is grinding. I took it apart and the gears looked like new and nothing looked out of place. When reassembled and re-oiled the noise is still present. I do not want to put on motor and run and have this lower unit fail if this noise is bad.

Both units were off motor when above symptoms occur. Old lower unit did not make any noise so that is why this lower unit concerns me. Any suggestions to try?

Thanks!

OK, what you are describing may be perfectly normal. Lift up a bit on the drive shaft when you turn it--is it smooth now? Then it probably is OK. Here is what is happening:

The weight of the drive shaft presses downward on the pinion gear. That forces the pinion gear into the forward and reverse gears and gives you that grinding sensation. However, when the motor is actually running, the thrust of the gears forces them apart, pushing the pinion gear and drive shaft upward. When the pinion gear is pushed up against the bearing above it, that is as far as it goes and the gear clearance is correct and the noise is gone.

Summing up, on a running motor, the pinion is forced up against the bearing, the forward gear is forced against its bearing, and the reverse gear is forced against the gearcase head. When all that happens, the gears will have proper clearance and be quiet. But when not running, the weight of the drive shaft mashes the pinion tight into the other gears, no clearance, and it grinds.

All perfectly normal.

 

[gobucs1]

Re: Lower unit grinding noise

F R Thanks!

When I slightly lift on the driveshaft as you said the noise does stop. Thank you for the such detailed response, I am not more knowledgeable about my motor. I was just concerned since my prior lower unit did not have such a noise and wanted to make sure before I had to buy a second replacement.

I will bucket test today if the weather gets any better.