Mercruiser alpha one knocking noise

[Weathersbytaylor]

Hello all, I’ve got a mercruiser 3.0/alpha one setup that’s been making a knocking noise. The engine is brand new and when I was installing the outdrive it was extremely difficult to install. My noise is only noticeable at idle and appears to be coming from directly behind the engine. I’ve also developed a leak from the rear main seal. Is it possible that an incorrectly adjusted engine angle could cause stress on the flywheel and cause a leak and a knock? I’m confident the engine internals aren’t the issue. Noise can’t be heard from outside the boat. Maybe a gimble ?

 

[alldodge]

Hello all, I’ve got a mercruiser 3.0/alpha one setup that’s been making a knocking noise. The engine is brand new and when I was installing the outdrive it was extremely difficult to install. My noise is only noticeable at idle and appears to be coming from directly behind the engine. I’ve also developed a leak from the rear main seal.

The drive should slide in very easy, and sice alignment was not done then yes your tearing up the drive, seals and coupler

Is it possible that an incorrectly adjusted engine angle could cause stress on the flywheel and cause a leak and a knock? I’m confident the engine internals aren’t the issue. Noise can’t be heard from outside the boat. Maybe a gimble ?

Yes, you need to adjust the front motor mounts with an alignment bar prior to installing the drive.

The Adults Only section (link in my sig) has info and how to's

 

[Weathersbytaylor]

Here’s the kicker, when I installed everything I purchased an alignment tool and it slid in and out very easy (two fingers easy) so I didn’t bother adjusting it. Would anyone happen to have a measurement of their alignment tool handy for comparison just in case I have an undersized alignment tool. Mine measures 1.009 on the end and 1.114 after the first step

 

[Weathersbytaylor]

My only other idea is that I have a bad/cracked coupler. I bought the boat with the engine torn apart so I never heard it run before the engine being taken apart. What are the symptoms of a bad coupler? I assume a knocking noise? Above 1200 rms it’s gone

 

[alldodge]

Here’s the kicker, when I installed everything I purchased an alignment tool and it slid in and out very easy (two fingers easy) so I didn’t bother adjusting it. Would anyone happen to have a measurement of their alignment tool handy for comparison just in case I have an undersized alignment tool. Mine measures 1.009 on the end and 1.114 after the first step

Wish you would have lead off with this, but your tool is dead on for the first two steps, the gimbal is the third step

 

[Scott06]

How high is the outdrive raised when you hear the knocking? If in full up trailer position knocking could be ujoints, or if you never ran it the u joints could be bad. Gimbal maybe, but if your sure it's inside the engine compartment and only at idle might be exhaust shutters

 

[Weathersbytaylor]

Doesn’t matter what position the outdrive is in. I hadn’t considered exhaust shutters. They were intact when I assembled the engine. I may pull off the exhaust and give that a try. Past that and coupler damage I’m at a loss. May replace the gimbal as a preventative measure.

 

[DouglasW]

Something like this maybe?
https://www.youtube.com/watch?v=7XKoYVTiVi0

 

[1985 Century Mustang]

Can anyone advise what this could be. Outdrive was removed after the video, and still makes the same noise. Coupler splines were fine also.