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Thermostat parts

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18Warrior

Contributing Member
Hi. I have a 1988 5.7 Mercruiser, with an Alpha 1, Gen 1 outdrive. It has a standard cooling system, design 2. I went through the parts list, and ordered the following parts for it:
25-8M0204561 O ring
8M0109441 160 degree thermostat
23-8M0214947 thermostat sleeve
My problem is that none of the parts fit! They are all oversized by about 1/8-3/16" too big in diameter. Were there two different versions of the same thermostat housing made??
I've attached the parts breakdown shown here on Marine Engine.
Any help would be appreciated
Screenshot 2026-06-26 10.36.03 PM.png
 
I don't have an engine serial number, all I have is the drive number. Which corresponds to the parts I looked up
 

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The key to getting the correct parts is the engine serial number. The drive serial number realy won't help you narrow down the range of the engine choices, especially if its not the original drive.

That vintage engine, with standard (RWC) cooling should have a 140 deg t-stat.

My guess is that you looked up an engine that's newer than what you have.

There should be a serial number plate above and behind the starter, on the flywheel cover. If its tight getting to that area, a cell phone in camera mode may be helpful.
 
There is actually a serial number sticker on the port valve cover, but the number is gone. When I cool off a bit, I'll go have a look by the starter for a plate.
 
18Warrior said:
I've read on a few different sites that the 160* stat will bring the temp closer to what the engines run better at, and give slightly better fuel economy. Remember, automotive stats are 180-195*.
Click to expand...
WRONG WRONG WRONG

AUTO vs MARINE for many things are not the same. DO NOT EQUATE these two in the future you will end up with serious issues.

The ONLY thing that is the same is the engine block and heads (Marine use Brass Core Plugs, Most Automotive use steel) and mechanical internals of the engine. (It is basically a Truck block) But everything that is attached to it is different including the on engine water pump/Circulating pump. Auto uses stamped steel one direction impeller, Marine used a Bronze BY-directional Impeller with a Stainless steel backing plate.

Now you know.

With a 140* thermostat the engine will run at about ~150* at idle and slow speed cruising and rise to about ~160-165* under high load or high RPM use

Your engine should be running a 140* Thermostat and it has to be STAINLESS STEEL. A 160* thermostat is used for specific applications which yours is not. For this vintage I would say a 160* is used with CLOSED COOLING. (The use of a heat exchanger and antifreeze in engine) Yours is RAW water cooled.

I have a 1987 350ci 260HP which is exactly the same engine. 140* Thermostat
 
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FYI, There is a reason for a 140* thermostat to be used in a RAW water Marine engine, It is to keep the salt in salt water suspended in the water to avoid corrosion.

Now there are many engine models with RAW water cooling that do use a 160* thermostat but yours is not one of them if it is RAW water cooled. (NO HEAT EXCHANGER)
 
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