I have seen this on several aluminum intake manifolds, including my own. Fresh water use, and the passages going into the heads look great, but there is significant corrosion and erosion of the aluminum in the flange area where the thermostat sits in the intake. I suspect this is being caused by galvanic corrosion from the stainless steel flange of the T-stat being in contact with the aluminum in the presence of water. Add to this the fact that there may be stray electrical current since the block is grounded and there are senders mounted on many of the T-stat housings. I am wondering if an aluminum restrictor would serve the same purpose as the T-stat and cut down on this corrosion. Either that or some kind of teflon spacer ring to go under the T-stat to insulate it from the aluminum. I know there are marine manifolds out there with bronze inserts, but they are hard to find in the popular aftermarket configurations, and they are quite expensive.

I'm wondering how concerned I need to be about this. Anyone else having this problem?

Look at having the water passages hard anodized. It will help this is what a lot of aluminum heads have done to them as well as aluminum exhaust manifolds. I'm not sure if it would eliminate a galvanic corrosion issue but it would slow it down.

Screwing a zinc in helps
Steve

Bud, theres a NPT type zinc that I have in my bellhousings with built in coolers. The old mercury style bellhousings. Im pretty sure its 3/8 NPT. But maybe installing one of those in a spare temp gauge port?

Heres a cheap one. Would have to trim it down

http://www.marinepartssource.com/new...E1F&mfg=CANADA METALS&mfgno=CME1F&desc=Zinc 3/8 in. NPT Pencil Anode

thats a good idea

I have a spare dual plane for Vortec SBC with stand run only time if your looking to save on that application.

They make the small screw in anodes with as small as 1/8" NPT. I have drilled and tapped into the ends of the heads and installed the 1/8" NPT anodes. You can install one in the front and rear of the heads, as well as adding an 1/8" NPT hole to the intake anywhere you would like. That's the nice thing about it. It's small enough so that you can add it almost anywhere.

You will still have some corrosion due to the 2 dissimilar metals, but it should cut down on it quite a bit.
Eddie

Good question and good info ..

I don't believe Zinc anodes are good at protecting aluminum in fresh water.
Maybe Magnesium or a sacrificial Aluminum/Indium anode.
http://www.performancemetals.com/anodes/AnodeFAQs.shtml

That's correct. Magnesium is a much better choice is fresh water.
Eddie