clogged passages in a D318 Marine

This is a problem with all the horizontal pony motors...it is a low spot that accumulates crud from the cooling system and about the only way you can get it out is pull off the pony heads and rod, blow, and high pressure flush it out.
What makes clearing more difficult is the connecting passage between the two cylinders do not line up.

OM, you are right those flat engines always were a problem with the coolant passages plugging up and I,ve seen lots of them plugged like that in fact not many that you took apart were clear but most did have some if not about completely plugged with junk like rust particles or just a plain dirt (sandy) type material like you see in the diesels when liners are taken out so is the same stuff.At the shop I always steam cleaned the junk out and the heads need to be steamed out too as they plug up too.

D318Marine - Is your engine running a heat exchanger, or just straight seawater cooled? The most effective, but rather harsh, cooling system cleaner is a solution of about 3% muriatic acid with some plain detergent added. The detergent improves the cleaning action of the acid by loosening rusty deposits that need a soap to assist with their removal .. and it also inhibits the undesirable, base metal attack properties, of the muriatic acid.

Formaldehyde is nowadays recognised as a rather dangerous and carcinogenic chemical, and is likely to be restricted in its availability.
If you use muriatic acid, make sure you indulge in the necessary safety steps of using gloves and eye protection, and ensure that the acid is not readily accessible to children or animals.

The most benign, but nearly equally effective cooling system cleaner is citric acid. Citric acid comes in granular form, looks like sugar, and is used as a food additive/preservative. It is basically 100% safe to handle and deal with .. although gloves are still advisable in case you have sensitive skin.
A 10% solution of citric acid will clean out rust from cooling systems and not attack the base cast iron or steel, or any other non-ferrous metals. Pressure washing is always advisable to clean out remote and hard-to-access cooling passageways.

Old Magnet. how do you clean the passages between the cylinders?

Ozdozer, most of the marine d318's were set up for keel coolers but mine has a heat exchanger and a raw water pump. In fact mine might be the only one with that setup in existence.

Could anyone tell me what the best antifreeze to use, and also should I put in a rust inhibitor?

I don't believe you could ever get every last bit of rusty crud out of blocks, unless you dismantled them to the last nut, bolt and bare component, and then hot-tanked them. You can only do so much, and then have to call it quits. Many passageways are just plain inacessible.

The biggest single problem is hard scale. Hard scale is difficult to remove and is also the greatest insulator against heat transfer. Muriatic works best on hard scale, but you have to balance the time spent (on acid treatment) against base metal attack. At some point you have to call it quits before the acid causes too much base metal damage, but when there is still some scale left. Getting 90-95% of the crud out, is what I would regard as an acceptable result.

There are many commercial brands of antifreeze with added rust inhibitors on the market. They are all basically similar .. propylene glycol with small amounts of added rust inhibitor chemicals. The RI chemicals are tailored to the engines they are designed for.
Many engines nowadays have high aluminum/magnesium alloy components, and these RI chemicals are designed to inhibit corrosion of these alloys, by inhibiting the galvanic reactions between the dissimilar metals.

As the D318 has little by way of alloy or non-ferrous metals in its construction, these "tailored" rust inhibitors aren't really necessary for the D318, and an antifreeze with basic cast iron RI's is all thats required. Any of the well known commercial brands of antifreeze with RI's should be quite adequate. The major rule is to change out your coolant every 3 years .. so you remove any contaminants, and degraded additives .. and replace the coolant with a fresh batch.

The heat exchanger looks like Monel. I'm not really sure if thats considered a non-ferous metal, but I can tell you it looks like it did when it was brand new.