Freshwater cooling for your Atomic 4 boat engine

So you have an Atomic 4 engine in your boat. The Atomic 4 is one of the most popular boat engines ever made for sailboats. There are thousands in existence.

However they came with raw water (lake/ocean) water cooling. This means that all of the crap/crud that is in the lake goes through your engine if it can make it through the raw water strainer, if you have one!

This means that your engine and exhaust manifold are corroding from the inside out. If you are in the ocean it is particularly bad since salt water only speeds up the corrosion process.


Freshwater cooling gets rid of these problems entirely. Plus, if properly maintained there is ZERO chance that your engine will be damaged by an early freeze or a poor winterization job.

Here is a picture from the Moyer Marine Forum. Moyer marine is keeping the Atomic 4's alive. They are a good source of parts and knowledge. They sell a freshwater cooling system but it is rather pricy at around $800. If you have the cash, buy it. If not read on.


I found a Universal Diesel engine heat exchanger on Ebay for just over $100. Its all copper and very nicely made.

I had to solder on some brass and bronze fittings to adapt it. Mostly I used 5/8" auto heater hose for the connections. However short pieces of 3/4" heater hose was required in order to use what fittings I could acquire from Menards, and to adapt to the Bosch Mag Drive pump. The Mag drive pump has 3/4" hose barbs.


This is a diagram of what I ended up with.



I reused the existing A4 impeller pump. It works just fine to circulate the raw water through the tube section of the heat exchanger. The rest of the system is filled with automotive antifreeze at a 50% solution. It took less than 2 gallons to fill the system.

The Bosch Mag Drive pump I purchased off Amazon. It is a seal-less pump, so there is no shaft seal to leak. The impeller is coupled magnetically inside the pump.

The Coolant fill tank is made for an Austin Mini Cooper. It has a 5/8" bottom hose barb and a 3/8" vent line barb which works perfectly. It comes with a pressure relief cap and its cheap at about $25.00 The Bosch pump was also purchased off Ebay for $89.00 and came with an electrical connector.

Search Ebay for:

Cooling EXPANSION RESERVOIR TANK With Cap For 02-08 Mini Cooper S R50 R53 R52

I also purchased the Bosch coolant pump off Ebay. Its Bosch part number 0392022002.


One thing that is not obvious is the vent line to the fill tank. Its important that the vent line connection attach to the coolant system at the highest point. That will allow air bubbles to rise to the top and be pushed back into the coolant fill tank. As you can see the coolant fill tank large 5/8" fill hose is just prior to the pump inlet. So some coolant constantly flows out the 3/8" vent line to the tank and then back down the 5/8 line prior to the pump. That keeps air bubbles from being trapped in the system.

On my boat, a Pearson 10M, the heat exchanger is mounted low on a bulkhead next to the engine. Pump sits on top of a tee that is attached to the heat exchanger shell outlet. The tee is where the Coolant tank fill line attaches. In the diagram, the pipe Tee strainer is shown prior to the pump, but it is actually prior to the heat exchanger so it will trap big chunks and prevent the heat exchanger from becoming clogged.


The Pipe Tee strainer needs some explanation.

It is constructed from a 1 1/4" PVC pipe Tee with a 1 1/4" NPT threaded socket on the side. The ends of the tee are solvent weld. I put 1 1/4" male by 3/4" NPT threaded bushings into the ends of the tee. Inside the Tee is inserted a stainless steel pot scrubbie. They are used to get tough baked on food off stainless and cast iron cooking ware. You can find them at the grocery store and cost about $2.00. Cram it into the tee. It will flow coolant just fine but keep flakes of rust and big chunks of dirt from clogging your heat exchanger and jamming up the pump. I didn't come up with this idea. Indigo Electronics mentions it in their FWC system installation manual. I suspect they borrowed the idea from some other place as well.

Indigo Electronics FWC installation

Indigo seems like another good supplier of Atomic 4 parts. But again, if you have the cash, you can buy their system.

Their is some debate over whether the circulation pump should be a mechanical pump or electric. The Bosch pump is cheap because it was used on hundreds of thousands of cars and trucks. Its likely that everything else will be more expensive. From what i can tell, most users of that pump get about 70,000 vehicle miles out of the pump. That translates to probably 1500 to 2000 hours of operation. That's a lot of engine time! Thats about 3 months of continuous operation. If you go with a mechanical pump, such as the one that Moyer sells you will need to change out the impeller every year or two. If you are paranoid about an electric, buy a spare and keep it on board. Its $89.00.


My Atomic 4 had a thermostat but I removed it. The housing was corroded even though my boat has never been in the ocean. So the thermostat would not have been very effective anyway. As it turns out with the plumbing shown, the engine operates at about 150 degrees under load. Not having a thermostat to worry about is one less thing to break. I think I will leave it out.



Bosch Water pump - 89.00

Heat Exchanger - $120.00

Fill tank - $25

Fittings and hose - $175.00

Total: $409


Here are pictures of the installation during testing. The fill tank will go outside of the engine compartment to the right. It could go inside the engine compartment but I want to be able to check the level easily and with its semi transparent sides that will be easy.