How do you deal with the black rust/scale (Fe3O4 aka magnetite) that forms in the water jackets? no one ever talks about this in engine builds - atleast anywhere I line that I can find. Engines were raw water cooled in the past, closed cooled now.

I have the water jackets spotless, excluding a thin (1/32 maybe 1/16) layer of black Fe3O4 clinging to the cast iron. I can barely chip it off with a chisel and hammer... I’m afraid I will damage the block doing this and certainly won’t be able to get to it all even with all of the freeze plugs out. Tried frayed cable spun in a drill, all types of pokers and scrapers, purple power, hot lye soak, pressure wash, evaporust, vinegar, electrolysis, nothing touches it. Oxalic acid is my last option without getting mid evil lol

i am concerned about overheating issues.

Muriatic Acid ,Flush with water then Neutralize with Ammonia and lots of water. Nasty as Fark , but it works . Just make sure and protect anything you dont want etched. If
it not flakeing off , leave it and roll on. It will be fine. Look into a passenger car block water jacket . they arent real clean either.

Sounds like you boat in real brackish water or salt water. Best cure is closed cooling.

you have to watch out what you use for a descaler especially if your running it thru the engines. Some of the acids are very hard on the T stat and its housing ---- also hard on the impeller and its housing ---- also hard on rubber cooling hoses and so on.. Then you have to worry about where are those harsh products are going if you are running them thru the engine.

Anyways there is about a handful of products that are used ---- some with good results.

Here is one

RYDLYME Marine: The Ultimate Biodegradable Marine Descaler! | RydLyme Marine

https://trac-online.com/product/barn...BoCd14QAvD_BwE

Well the blocks are bare, so I can try anything (and have). I took a look at them tonight and the scale is NOT going to come loose, so as long as it doesn’t cause heat transfer issues it will be ok I suppose. Anyone have pictures of water jackets in a raw water cooled engine that isn’t having overheat issues I can compare mine to? I don’t typically build blocks that were raw water cooled but I had these, now I’m second guessing myself.

here’s a pic of the scale:

I would be more concerned with the pitting on the cylinder walls than the oxide layer.
They look damn clean to me.

Same here. Those are some pretty deep and big divots. I have seen the black junk before and that isn't what yours looks like. Looks like corrosion from salt water.

The pits are in the black corrosion layer, up to 1/16" thick in places. The middle of the pits is exposed iron where I was chipping it off with a punch and hammer, its magnetite so its basically like chipping a thin rock/stone layer off. In the areas I have been able to get the black layer off completely the iron is not pitted very badly. I will double check the pits and wall thickness in those areas.

Here is a picture of that same spot now that I have them as clean as I can get them. This is how all of the surfaces look.