Engine bed: Fix mounts to aluminum bar - will it work?

[boatpoker]

Originally Posted by Dieseldude

As an engineer, I am aware of properties of materials and cost vs. benefit and performance. Of course engine components are designed with compromise considering such factors as cost, strength, heat resistance, durability, mass, etc. That being said, the greatest mass of material that most small to medium size cruising boat engines are made of is cast iron. This would be followed by steel, then perhaps aluminum. Then smaller amounts of various other alloys. Stainless steel is generally one of the least used materials in boat engines.



Ideally, engine parts are made of the best available materials according to conditions that they must endure. But compromises must be made, and often cost is a big consideration. Not sure of the point of your question. Example: It might be great to have a stainless steel oil pan, as it sits above a bilge that can hold sea water that can slop over it. But the cost of one would be several times that of mild steel. So the manufacturer uses mild steel to keep the cost competitive.



The question of using mild steel or stainless for mounts is comes down to cost and convenience, as both are of sufficient strength and durability.

Stainless steel is a wonderful material but it is nether stainless (it should be stains-less) nor ideal for many applications. As a retired Certified Marine Corrosion analyst I found it inappropriate for this application with the constant vibration and galvanic issues caused by steel bolts and mild steel embedded plates. Galvanism is measured in millivolts, it does not take much of a difference to cause corrosion. Anyone who believes these never get wet has not done much cruising. You can choose what ever you like. My experience with thousands of surveys and marine corrosion training directed me to mild steel.

[Dieseldude]

Originally Posted by boatpoker

Stainless steel is a wonderful material but it is nether stainless (it should be stains-less) nor ideal for many applications. As a retired Certified Marine Corrosion analyst I found it inappropriate for this application with the constant vibration and galvanic issues caused by steel bolts and mild steel embedded plates. Galvanism is measured in millivolts, it does not take much of a difference to cause corrosion. Anyone who believes these never get wet has not done much cruising. You can choose what ever you like. My experience with thousands of surveys and marine corrosion training directed me to mild steel.

Actually, my first reply on this thread said the same thing. I recommended mild steel, as I did not expect much exposure to sea water. But someone made the point that repainting might prove difficult. The engine may need to be lifted. This made stainless seem to be more convenient. I understand concerns about galvanic corrosion. But for the application, I would have expected the material to be dry most of ts life. I have seen stainless fitted next to steel in various applications without much ill effect. A classic example is a stainless impeller in a cast iron sea water pump housing. It seemed to be ok though.

[Bowdrie]

To the OP, there is nothing wrong or to fear from doing what your diagram shows in the very first post in this thread.
Four engine mounts with each having two bolts tapped 5/8in into aluminum will never cause any consternation of pulling out even if the boat turned upside down.
Use mild steel if you wish, SS if you feel it's worth it, it's not going to the moon and it's not rocket science.
No matter what you do, between the original embedded steel, the spacers, and the mounts and bolts there will be some mis-matched metals.
Slather some NIKAL anti-seize on everything, wipe off the excess when all is fastened down with engine aligned.
Good to go for many years.
We've had 4 pages, (me included,) of feverishly trying to make a mountain out of a molehill, (snatching defeat from the jaws of victory).