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How to Replace a Lawnmower's Spark Plug

The spark plug is what makes it possible for your lawnmower's engine to ignite the gasline during the combustion process. And while a clean spark plug allows for easy starting and efficient engine operation, a spark plug that's old, dirty or fouled won't be able to deliver the needed spark for the combustion process within the engine's cylinder. If your mower is difficult to start, a worn-out spark plug may be the culprit.

Step-by-Step Plug Replacement

Replacing a spark plug is one of the easiest tasks of lawnmower maintenance. The only tools you'll need are a ratcheting socket driver, a spark plug socket and a spark plug gauge. Follow these simple steps and you'll be on your way in no time to a properly running lawnmower.

  1. Disconnect the lawnmower's spark plug lead; then clean around the spark plug to remove any debris that might have otherwise fallen into the spark plug hole once it's removed.
  2. Use a spark plug socket to remove the spark plug. These specialty sockets have rubber inserts to protect the spark plug's ceramic body, and they're available in a variety of sizes to fit any size spark plug (13/16 and 3/4 in. spark plugs are the most common). If the spark plug refuses to turn, spray it with a penetrating lubricant and allow it to soak in for about 10 minutes.
  3. Once the spark plug is removed, inspect its electrode to see how the engine is running. If the spark plug's electrode is wet, there may be a problem with the engine's choke system or the fuel mixture may be too rich. If the electrode is very dry and powdery, the engine may be starved for fuel or there may be a crack in the carburetor mounting gasket.
  4. Check the gap on the new spark plug before installing it. A new plug will often come pre-gapped to match your specific engine, but use a spark plug gauge to verify that it matches the manufacturer's specifications. If necessary, use the gauge to gently bend the curved electrode to the correct gap. You'll know the gap is correct when the gauge slightly drags as you pull it through the gap.
  5. Insert the new spark plug into the spark plug hole, hand turning the new plug until the threads catch. Switch to the spark plug socket and ratchet the plug down until it stops; then turn it another quarter turn. Do not overtighten or apply too much torque to the spark plug, as you'll either crack the plug's ceramic body or you'll make it next to impossible to remove the plug the next time.

While all spark plugs are printed with a model number on their ceramic bodies, it's not uncommon for the printing to wear off over time. If you can't read the spark plug model number, or if you're unsure as to what fits your lawnmower, check with a service professional. He or she will be able to quickly determine which spark plug you need, so that you can easiiy get your mower back in service.