Replacing 143* thermostat on 120 Mercruiser I/O 53 year old Sea Ray

Paul E Meyer

Paul E Meyer

New Member
Apr 25, 2020
5
Dayton, Ohio
Boat Info
1967 SeaRay SRV170 4cyl 120hp Plus, I am the ORIGINAL owner of this SeaRay.
Engines
153 cu. in. 120 hp Mercruiser 2bbl
Hello,
I am the original owner of a 1967 Sea Ray I/O SRV170 153 cu.in. L4 120 HP engine. I need to know how to replace my water thermostat. Specifically, which end goes up. When I winterized it last fall, I wondered if I have been installing it incorrectly for years since it will "fit" either way into the block. Does anyone know how it goes. Look at photos below and let me know which way is correct. Is it spring up, or spring down? Please, no guessing. Thanks.
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Jeeze I really had to think about that. The spring should go toward the engine so that when closed it heats up with the engine af and expands opening the valve. I can’t tell what the orientation should be in your pictures because I can’t see the heat exchanger.

How about some pics of the boat? You may have one of the oldest Sea Rays here on CSR.
 
Thanks for your reply. I have included a photo of the thermostat that I have been using for perhaps the last 15 years. It may likely not be a genuine mercruiser replacement part. Its design prohibits inserting the spring down because of the side pieces. It would not fit down. So, all these last 15 or so years, I have mounted this thermostat with the spring in the up position. When I decided to replace it last fall, I wondered if I had been using the correct thermostat since the replacement that I got will fit either way. The boat performed well with the thermostat spring in the up position with no indication of engine overheating. But, I decided last fall that I want to know once and for all which way should the spring go. I have also included a few more engine photos that Henry requested so he could make a determination.
Finally, I also included a photo of my boat that was purchased brand new in June, 1967, in Dayton, OH. (along with my '67 GTO.
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Paul,

There are two hoses in your picture. A single long one that is almost above the pulley (water pump?) “A” that appears to be attached to the base fixed to the engine . There is a short hose that seems to feed the thermostat Cover “B”. Finally there is a smaller diameter hose “C”. Where do they all go?

Which hose goes to the heat exchanger? A or B? From the pictures it looks like C goes into the Tstat housing and accesses the area under the thermostat. What else feeds that area under the thermostat? Is it the engine block?

I’m really perplexed at how the water flows. A and B allow flow that does not go through the thermostat. I’m also perplexed as to why the thermostat can go either way and not overheat. Put the thermostat upside down in your GTO and it will overheat in minutes. Curious
 
The old thermostat was installed with the large spring up and enclosed into the bolt down "cap." I guess the old thermostat was designed such that it would not fit spring down so there was no way to install it incorrectly. In any event, I assume that if the thermostat was installed incorrectly, I would know fairly quickly because the engine would overheat.
 
Paul,

The spring is the temperature measuring element. Liquid flows past it and as the liquid warms the spring it expands and pushes the valve disk away from the thermostat housing allowing water to flow through the opening.

On an automotive engine like your GTO cold water is picked up from the bottom of the radiator into the water pump. The pump pushes it into the block and it moves through the block to heads and intake manifold. The thermostat is mounted in the intake manifold spring down. As the engine heats up the water, the spring expands, opens and allows the water to flow to the radiator.

We know if the thermostat is installed spring side up it will cause the automotive engine to overheat fairly quickly because there is very little to no coolant in the top side of the housing.

Your boat engine had the thermostat mounted “upside down”, spring side up.

Boat engine thermostat has major water flow served by large hoses (2”?) with flow past the spring. This has no overheating issues.

Water flow in the boat engine is not clear. Also what does the little hose do? You don’t have a water heater.

The above tells me that the system works with the thermostat spring side up. So keep it that way. I can’t tell you why, much as I wish I could, it is a real head scratcher.
 
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From manual 1 need a burn

click between the: click to enlarge and from manual1
 
Thanks to both of you for your information. Bt's illustration clearly shows the thermostat mounted in the up position--like mine has been. In fact, the illustration shows a thermostat exactly like my old one that is designed to be installed only one way because of the side pieces preventing it from lowering the spring end into the block.
That photo showing one small hose with the white "collar" is the fuel line.
I will reinstall the thermostat with the spring up and I should be good to go. Can't do much else this summer so I hope to get in some serious boating.
BTW--Sea Ray Living Magazine featured my boat and me in a story in the Spring 2009 issue with Alan Jackson on the cover.
 
Paul E Meyer said:
Thanks for your reply. I have included a photo of the thermostat that I have been using for perhaps the last 15 years. It may likely not be a genuine mercruiser replacement part. Its design prohibits inserting the spring down because of the side pieces. It would not fit down. So, all these last 15 or so years, I have mounted this thermostat with the spring in the up position. When I decided to replace it last fall, I wondered if I had been using the correct thermostat since the replacement that I got will fit either way. The boat performed well with the thermostat spring in the up position with no indication of engine overheating. But, I decided last fall that I want to know once and for all which way should the spring go. I have also included a few more engine photos that Henry requested so he could make a determination.
Finally, I also included a photo of my boat that was purchased brand new in June, 1967, in Dayton, OH. (along with my '67 GTO.View attachment 83200 View attachment 83201 View attachment 83202 View attachment 83202 View attachment 83201 View attachment 83201 View attachment 83199 View attachment 83199 View attachment 83203 View attachment 83203
Click to expand...
Very nice!
 
I would love to see a photo of your boat. You've got my 72 beat by 5 years. Yes the diagram clearly shows the spring pointing up. Why they did it that way, who knows. On mine it points down (a mercruiser 165)
 
Nice looking boat
 
Actually the antifreeze in the photo is just put in the block for winter storage to prevent rust. It is cooled with plain old lake water . . . BTW the engine has never been overhauled or removed from the boat.
 
Paul E Meyer said:
Actually the antifreeze in the photo is just put in the block for winter storage to prevent rust. It is cooled with plain old lake water . . . BTW the engine has never been overhauled or removed from the boat.
Click to expand...

That makes more sense. The complicated thermostat housing allows a constant flow of fresh water to flow through the exhaust manifold. The engine water pump circulates water through the engine block creating a positive pressure within the engine block. The pressure differential between the block and the fresh water flow diverts the fresh flow to exit to the manifold. When the thermostat opens it bleeds hot water into the ongoing stream to the exhaust reducing the pressure within the block in turn drawing fresh water into the block.
 
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