Kubota (Nanni) knocking noise

[Ostinato]

I have Nanni 3.100HE diesel (marinized Kubota) which is making a distinct knocking noise when in forward gear at low pm. The engine is attached to Twindisc SP60 sail drive. The drive has less than 200 hours; the about Nanni about 2400 hours.


No similar noise is heard in neutral or reverse. It does not matter whether the engine is warm or cold. The noise disappears as the RPMs increase.



I had a mechanic look at it supplied by Twindisc that said it was not the sail drive but either the coupling plate (dampening plate) which also has about 200 hours or a rod. My oil pressure is good.



I am at a loss to understand why the noise is only in forward gear. Anyone have any trouble shooting ideas? My first option is to change out the coupling plate but after that I am stuck.

[a64pilot]

Well a rod should not care about direction and gets worse under load and when warm, is usually accompanied by low oil pressure and often an overheating problem.
However I think I would cut open the oil filter to further make myself feel better, but I don’t think it’s a rod.
The loudest, worst sounding knock I have ever heard was on a pickup truck with a broken flex plate, so they can make a terrible rod sounding hollow knock.
However on that one it was present all the time.

[jamhass]

Engine mounts.

[shakey doug]

Had a job once to check out the same issue with a perky 4.108, ran through all the usual checks engine, mounts, transmission etc, problem only showed up in Ahead only, recent work done 125hrs previous a new age synthetic drive plate fitted. Removed transmission then Drive plate to reveal a prominent polished area on the Flywheel, removed 3mm off plate centre boss, checked transmission to adapter alignment. All reassembled and still running quietly many hours and 8 years later. My take on causes. 1. Synthetic drive plate softened allowing flexing after a few hours use. 2. Neutral, no torque no noise 3. Propeller, not very efficient in reverse, not enough torque transmitted to deform plate.