Taking Care of Your Mercury 2-Stroke Pt. 1: The Thermostat

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This will be a multiple part series about how to take care of your aging early model 2-stroke outboard. Check back to further articles on other yearly maintenance items you can easily do yourself.

If you’re anything like me then you’re very reluctant to let go of your trust-rusty 2-stroke outboard. My Mercury 40HP 2-stroke is a 1999 and has been purring along nicely over the years just fine. What’s my secret? Maintenance, of course! Every year there’s a few simple maintenance items you can do on your 15-60HP 2-stroke motors from various companies like Yamaha, Mercury, Johnson, Nissan, and etc that can extend the life of the motor exponentially. This series will cover the exact steps for a 1999-2004 Mercury 40-60HP 3 Cylinder 2-stroke but some others might be very similar.

My motor is never hard to start, never dies out at idle, never coughs, and barely smokes- all because I keep it properly maintained through out the year with just a few simple maintenance items.

In this part one of the series I’ll cover two basic things you can do to your 2-stroke that will keep it running clean and cool- the thermostat.

What You’ll Need:

  • Socket Set or Applicable Box Wrenches
  • 3/4″ Ratcheting Socket Wrench
  • Applicable Thermostat Gasket

The DIY:

Step 1: PourĀ a cup of sweet tea.

If you don’t like sweet tea you’re a communist or a yankee and should therefore just take your outboard to a local shop to have them fix it for you.

On to step 2…

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Step 2: Remove top cover and locate thermostat.

On the Mercury 40-60HP 2-stroke outboards the thermostat is this thing in the top right of the port side of the outboard.

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Step 3: Remove four boltsĀ holding thermostat to the block.

See the four bolts holding the thermostat onto the block? Remove them with your socket and ratcheting wrench and/or box wrench that fits them…I forgot what size it was.

BE CAREFUL here as some bolts may be seized into the block if the thermostat was never services. When in doubt go slow with even pressure to remove them for the first time- we’ll solve this problem for the next service once they’re out of there.

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Step 4: Remove thermostat and its black plastic housing from the block.

At this step you’ll also figure out if you need a new thermostat gasket or not. I have replaced mine several times over the years so sometimes mine comes off without breaking apart. If yours crumbles like a badly built….something or other…just scrape it off and apply the new gasket in the proper orientation when we replace the cover.

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Thermostat removed and cover removed.

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Notice all of the garbage stuck in my thermostat. Tons of small snail shells and dead grass weeds.

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Step 5: Clean thermostat.

Self explanatory- the reason we’re doing this. TA-DA! So fresh and so clean, clean.

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Step 6: Re-assemble thermostat assembly in reserve order of what you just did.

If all went to plan above you should now have a clean thermostat and etc. that’s ready to reassemble. Put the thermostat back into the black plastic housing, shove it into the block, and line up your new gasket if necessary.

Take this time to apply some teflon grease to the screw threads. This will allow your to easily remove these screws in the future when doing this maintenance again. If you are retorquing the screws to the factory spec of 120 lbs, remember that grease will lower your necessary torque value! Don’t over-do it!

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It should now look like this again, and you’re done! Put your engine muffs and hose connection on the lower unit and try it out to ensure proper operation.

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All done.




2 thoughts on “Taking Care of Your Mercury 2-Stroke Pt. 1: The Thermostat”

Comments are closed!

  1. Jigalode says:

    Simple, yet, great concept for a blog series. Can’t wait for the rest of them.

    1. matt says:

      Yup! Got more coming…spark plugs, carbs, and water pump!

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