Shaft rotation in neutral

[PAllen529]

I’ve got a pair of Cat 3406E engines. Today I noticed that the port engine shaft rotates slowly at idle and in neutral. I would guess it to be maybe 8-10 RPM. I can stop it by hand and hold it from moving but when I let go it starts again. I tried shifting forward and reverse a couple times but it continues. Somehow this sounds expensive. Ideas?

 

[Comodave]

Neighbor had that happen to his boat last summer. They had the transmission rebuilt this winter. It is supposed to fix it.

 

[DavidM]

Assuming the transmission oil drain plug is magnetic, I would check it for metal particles. But it may just be the oil dragging the output shaft. If it goes back and forth into gear solidly, I wouldn't worry any more if the drain plug is clear.

David

 

[hmason]

I’ve had that happen when docked in a current.

 

[psneeld]

A bit more info...

The shaft turns freely when engine is running and in neutral? Completely normal for many transmissions.

The shaft rotates with the engine off but even in fwd and rev it rotates? Completely normal for some transmissions.

 

[diver dave]

Slow rotation of marine wet clutches in idle is usually caused by warping of the discs in the clutch pack. And that is usually caused by overheating, and that is caused by slipping at high torque, and that is caused by low fluid, or more rarely low oil pressure or worn friction discs.

https://www.diva-portal.org/smash/get/diva2:991361/FULLTEXT01.pdf