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Installing a Heat Exchanger

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I’ve written this post to perhaps try to help others who are trying to install a fresh water cooling system. I’m by no means a mechanic. Quite the opposite, I sit in front of a computer all day making maps. It took me much longer than anyone with a hint of mechanical background, but I did get it done with just a few problems.

We were lucky get a heat exchanger included from the previous owner although it was not installed. After calling around I was quoted $1,600 for a complete new kit. I couldn’t believe it. After making numerous calls to local mechanics, I figured out the heat exchanger I have will work, albeit it needs a few replacement parts. I ordered most of them Friday which turned out to $170. Not bad considering the previous quote. I doubt that will be the end though. I usually end up running ot the store five more times before any job is complete.


Here are some pictures of what I was starting with. A normal raw water cooled 5.7 Litre Mercruiser.

Detailed Diagram of 5.7 Litre Mercruiser Raw Cooling Setup



I took my existing, old, heat exchanger looked at by a few boat shops around town. A few told me to just buy a new kit ($1,600) others told me I was working with garbage and I should buy their (garbage) used heat exchanger instead. In the end, I found a great local boat shop, called Mercury Marine that maintains the Prince of Whales fleet. The owner is fantastic and hooked me up with some Heat Exchanger end plates, expansion tank, gaskets and thermostat.

Here is the heat exchanger before I started. It was missing end plates but had most of the hoses and clamps. The inside of the heat exchanger is fairly clean although it looks a bit beat up on the outside.


Heat Exchanger Parts

There have been about 5 different trips to various parts stores to complete this heat exchanger. Its been tedious to say the least. I have no idea how a mechanic would go about this. I can’t imagine they would be able to bill all of their time for finding parts. Anyways, I now have the following parts all ready to be put together:

Mounting Bracket

I had to buy a new mounting bracket. The bolt pattern of the one I had didn’t line up. The one pictured below gets bolted to the thermostat port.

160 degree Thermostat

While I was at it, I might as well replace the thermostat. This guy cost me around $10. The genuine Mercury equivalent would have been over $20.

Thermostat 5.7 liter mercruiser

Heat Exchanger End Caps & Gaskets

Pretty self explanatory here. These guys get bolted onto each end of the heat exchanger.

San Juan Heat Exchanger End Caps

Mercury Reservoir Tank Kit

Pretty straight forward.  

Mercruiser Coolant Resevoir Tank


Dry Fit / Wet Fit

Now that I have all of the parts, I went ahead and put everything together prior to bolting everything together. I check for clearance, bolt sizes, gasket sizes and anything else.

Mercruiser Heat Exchanger Dry Fit


I later moved the location of the reservoir tank to be higher. I’m still struggling with this one as I’ve read that bubbles coming out of the heat exchanger need to be able to float up and into the reservoir tank otherwise you end up with bubbles in your fresh water cooling system.

Mercruiser Resevoir Tank


End Caps installed on heat exchanger


Mercruiser Heat Exchanger



Once I determined everything fit, I went ahead and bolted everything together. Made sure hose clamps were secure and started her up. Everything seemed fine right off the bat. No leaks and proper flow throughout the system. Temperature climbed to about 130.

Heat Guage Sea Ray 230

Screw Up

Later while I was putting my tools away I found a brand new Stant Thermostat. A quick search and I found the old thermostat housing as well. Obviously I had completely forgotten to install a thermostat of any kind. Not too smart. So off I went and completely drained the system, pulled off the thermostat housing and put in the new Thermostat. Upon letting the engine run, the temperature quickly climbed to 170 and leaks quickly showed up at a few hose clamps and the Rad cap. It quickly dawned on me that my prior test had not tested the system under high pressure as everything was essentially still cold.

After installing a new Thermostat cap and tightening a few hose clamps, I was good to go. No big deal but glad I found out prior to putting the boat in the water.


Helpful Links

The 1993 Sea Ray 230, 5.7 litre uses this San Juan Engineering kit.

Torque specfications and Schematics


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