How to Replace Spark Plugs on Mercury 20HP Outboard

Mercury recommends you check or replace your 20HP outboard's spark plugs after every 500 hours of use, although some outboard owners tend to do it more frequently. 

Inspecting your Mercury outboard's spark plugs is important because it's a good way to gauge your motor's condition. Rather than reinstalling the old spark plugs, some people prefer to change them upon inspection because they're so inexpensive to replace.

Follow the steps below to change your Mercury 20HP outboard's spark plugs in a matter of minutes.

Tools and Parts Needed - Mercury Outboard Spark Plug Replacement


  • 3/8th ratchet
  • Ratchet extension
  • Torque wrench
  • 5/8th spark plug socket

NOTE: These plugs are for the 2016 Mercury 20HP 4-stroke outboard model. Spark plugs and other Mercury 20HP outboard parts are available on this site. 

Mercury 20HP 4-Stroke Spark Plug Change

Step 1. Remove the engine cowling by releasing the two latches. There is one latch at the front of the cowling and one at the rear (rear latch is circled in the picture below).

Mercury outboard spark plug change

Step 2. Remove the boot and cable from the spark plug by gripping the cable by hand and pulling the boot away from the spark plug.

PRO TIP: There are two spark plugs on a Mercury 20HP 4-stroke outboard, and it's important that the cables are attached to the correct spark plug. Label each cable before removing them both, or remove one cable, and replace that spark plug and its cable before working on the other spark plug.

Mercury outboard motor spark plug change

Step 3. Remove the old spark plug using a 5/8th socket, extension bar and ratchet.

Step 4. Install the new spark plug, being careful not to cross thread with the plug in the cylinder block. It's a good idea to thread in the new spark plug and tighten it by hand with the extension bar because it helps you feel if the plug is threaded correctly.

Mercury outboard engine spark plug change

Step 5. Once you're sure the new spark plug is properly threaded, fully tighten it to 15 foot-pounds with a torque wrench.

Step 6. Replace the spark plug boot and cable. Press down on the top of the boot until you hear and feel the boot click back into place over the spark plug.

Repeat steps 3-6 if you're replacing the spark plugs one at a time. 

spark plug change mercury outboard

Step 7. Replace the engine cowling and you're done!

Got a higher horsepower Mercury outboard engine? Here's how to change the spark plugs on a Mercury 115HP outboard.



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