Galvanic corrosion of aluminum cylinder heads.
 

SBC with aluminum intake and cylinder heads. Blew a head gasket due to galvanic corrosion. Had heads repaired but now need to make sure it doesn't happen again. I don't have a mercathode system. I did use pencil anodes in each
head but obviously that didn't work. I have since taken cable and grounded each cylinder head to the battery terminal. The block has always been grounded.
I was thinking on taking a ZINC hull anode and bolting that to the transom and running some ground cable to it from each cylinder head, trim tabs and out drive. Would that work or does it have to be in water?
I do have anodes on my drives which I change yearly.

Any advice would be appreciated.

are you running in salt water?

Where on the heads did it corrode? Was it where the small holes are on the bottom of the block near the head bolts but there are no holes in the heads?

Reason I ask is a few years ago I rant inot that problem on an SBC I was building. I had the heads fixed but when I built the engine I custom ordered a cosmetic head gasket with those holes deleted from the gasket since they are unused anyway. Then it couldn't corrode in that area.

I am running in Saltwater and it was those little hole in front of the cylinders that caused my issue. I tapped the holes and Loctited small plugs in them. Problem solved. The corrosion was primarily around those wholes and it was like it was trying to bore through the aluminum heads. There is generalized pitting across the head surface and intake side also. The intake showed same amount of small pitting and corrosion too. Now that everything is fixed, I want to make sure those heads aren't attacked.

Maybe helps if you use more active anode. Put every plug and hole in engine water jacket piece of magnesium bar. They will last maybe less than 1/4 season and must replace often but more active anode protect aluminium parts better. Maybe it helps if you increase zink anodes in water jacket, they last better but protect worse. Always anode will be killed, fast or slow. Magnesium is only way to go in fresh water, sink won't work at all. Hope you get the idea.

Thats sorta what I figured. I ran into the same problem and it was a salt water engine which was regularly flushed. I think those passages are so small that salt water gets trapped up in there and even when flushing it can't flush it all away. I also had Cometic delete the two upper water holes that don't go into the head. Then the combination of salt water, engine heat and two dissimilar metals, its a ticking time bomb.

It's been two years since I built that engine and haven't heard a word about it from the owner so I think I solved the problem.
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Here's the Cometic info I have from back then.

Closed cooling will completely solve it and should be considered.