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How To Replace Lawn Mower Spark Plugs

Every year you should change the spark plug in your lawnmower as part of annual maintenance. Changing out a spark plug in a two or four-cycle engine is similar to replacing spark plugs in your car. Lawn mower engines fall into two basic categories: four cycle and two cycle. Four-cycle engines depend on a crankcase for oil lubrication. Two cycle engines run on a fuel/oil mixture that supplies lubrication. Both engine types have at least one thing in common; they use a spark plug to provide the ignition for combustion.

Remove the old spark plug

Replacing the spark plug in your lawn equipment is a good idea each year

Replacing the spark plug in your lawn equipment is a good idea each year

  1. In many models of lawnmowers or other types of gas-powered lawn equipment, you may have to remove a shield or cover first to gain access to the spark plug.
  2. Disconnect the plug wire from the spark plug terminal nut.
  3. Using a spark plug socket and ratchet or the proper size open or box end wrench, unscrew the old spark plug.
  4. Inspect the old spark plug for excess oil, a broken insulator, or a burned or missing center or ground electrode. Finding any of these conditions indicates a problem in the engine that should be addressed before further use.
  5. Use a spark plug adjustment gauge or gap tool to set the gap of the new plug. Most new plugs are supposed to be factory set to the proper gap but this is not always the case. It is best to verify that they are set properly to avoid further engine malfunctions.
  6. When setting the gap, use a proper gap gaugeā€”not a screwdriver, knife blade, or pair of pliers. You could damage the insulator, the center electrode, or the grounding electrode.

Install the new spark plug

  1. Apply a good quality lubricant to the threads of the new spark plug. Engine oil works well. You can also use anti-seize lube but use it sparingly.
  2. Clean all the dirt from around the spark plug port in the engine.
  3. Screw the plug in hand tight, then give it 1/2 to 2/3 more of a turn. For tapered seat plugs, give it not more than 1/16 additional turn.
  4. Reinstall the spark plug wire.


If you cannot remove the old spark plug, run the engine until it heats up slightly, then try to remove the spark plug again while the engine is hot.

Make certain that you buy the precise spark plug for your particular piece of equipment. Do not substitute one type of spark plug for another. Even though they may look the same, the substitute may damage your engine.

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