How to Change a Spark Plug on a Lawn Mower or Trimmer
A spark plug produces the electrical spark that ignites the air-fuel mixture in the combustion chamber. Plugs wear over time: the electrode gap widens as the electrode material erodes, and deposits from combustion accumulate on the ceramic insulator and electrode surfaces. A worn plug misfires, starts poorly, runs roughly at idle, and reduces engine efficiency. Annual plug inspection and replacement when needed is one of the simplest and most effective maintenance steps for any gas outdoor power tool.
Tools Required
A spark plug socket in the correct size (typically 5/8-inch or 3/4-inch), a ratchet with extension, and a feeler gauge or spark plug gap tool are the only tools needed. Spark plug sockets have a rubber insert inside that grips the ceramic insulator and prevents it from cracking during installation. A standard deep socket does not provide this protection and risks cracking the ceramic.
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Reading the Old Plug
The condition of the electrode and ceramic on the removed plug provides useful diagnostic information about how the engine is running before the new plug goes in. A normal plug has light tan to grey deposits, a slightly rounded center electrode, and no damage to the ceramic. Replacing the plug on a normally operating engine is maintenance. Replacing a plug that shows lean burn (white ceramic, bluish discoloration), rich running (black carbon), or oil fouling resolves the immediate starting problem but does not address the underlying cause. Fuel mixture, air filter condition, and oil control ring condition are the areas to investigate if the plug condition is abnormal.
Replacement Interval
Most small engine manufacturers recommend annual spark plug replacement for residential use equipment, or replacement when inspection shows worn or fouled electrodes. The cost of a replacement plug ($3 to $8 for most residential small engine plugs) is low enough that annual replacement at the start of the mowing season is a straightforward preventive step that eliminates plug condition as a variable in any starting difficulty that arises during the season.
The full seasonal startup maintenance sequence, including plug replacement alongside air filter service and oil level check, is covered in the lawn mower maintenance guide in the lawn care section.
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