The V8350's have there flywheels replaced every 200 hours as part of normal maintenance, id imagine your 300's would be in the same boat being the same block. Theres lots of info on the issues on The hull truth.

The earlier 300 V6's also had a thrust bearing issue, ive heard that the later ones are better when they changed the design. My 300 yammy always had heaps of salt crusted on the engine block and behind the leg casings, if you didnt keep on top of it id imagine there could be issues with corrosion in the future. 

Next boats going to have Suzuki's....

 Far out I was just talking to someone yesterday who's brother's boat had the same issues.

jeffi know your boat is a sea rescue rig but do you guys flush the motors after each use , also are the motors left in the water or do you have a dry dock.   Just interested  

Sits on an airberth and almost impossible to flush the motors without standing in a dinghy or being Jesus Christ. The engines are raised up so nothing is in contact with the water. No battery chargers or external power is connected as well.

Doesn't get away from the fact that the anodes in the manifolds are like new yet the ali has eaten through?

 doesnt it have a flushing port on them??

 Hi jeff we have a similar issue with our twin 350 on the Niad.  So one of the guys made up extension tubes and non return valves on the flushing ports so we can stand at the transom and connect a hose to easily fresh water flush each motor for a set time after each use to extend the life of the motors ,  this doesn't explain the water damage to the manifolds which is another issue which needs to be adressed no doubt 

I take it its not the case - but is there a chance that something such as Inox or other corrosion inhibitor has gotten onto the anodes

Jeff one thing you could look at is how low the motors sit in the water and when the boat is reversing at low rpm or when the boat is slowing down from reasonable speed does the wash behind the boat splash up against the exhaust ports and possible water entry to exhaust manifold, also I have been told the older exhaust manifolds use to go down to the bottom port and turn in , so any water in the manifold would drain into the bottom port ,where as the newer style manifold drops past the bottom port by 10 /15 mm to prevent this from happening.

hi John. , sorry about that , it was Jeff who I meet few years back at Exy crossed lines , interesting corrosion from inside , is the problem with both motors .?? I assume you have a twin rig

Corrosion of aluminum in the passive range is localized, usually manifested by random formation of pits. The pitting potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting.

For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl -) is the most frequently encountered in service. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential.

Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions is considerably more noble (cathodic) than in halide solutions and it is not polarized to these potentials in normal service.

So sounds like the anodes aren't connected to the heads properly?

Could also be a missing earth, gaskets on the manifold could prevent electrolisis ( spelling? ), seen it happen on bigger boats when a wire snaps.

Wired up correctly and for it to occur on both engines??