Not all spark plugs are created equal, so always follow the engine manufacturer's recommendation for a replacement spark plug. Not long ago, there were fewer types of plugs available for air-cooled engines and installing the wrong one might not have necessarily done any major damage over the short term. It's a different world today. Here are some general guidelines for proper spark plug replacement:

  1. Start with a cool engine. Steel and aluminum have different rates of expansion. This can cause a steel plug and aluminum cylinder head to bind together and damage the threads of the softer material.
  2. Gently twist the boot off. Grab the spark plug boot and gently but firmly twist left and right while applying outward pressure. Never pull directly on the wire.
  3. Clean off the debris. Blow the area around the spark plug clean with compressed air to prevent any debris from entering the engine when the plug is removed.
  4. Select the correct spark plug socket. Modern engines may use ⅝-, ¾- or 13/16-inch sockets.
  5. Remove the old plug. Set it aside for later inspection.
  6. Select the correct replacement plug. Refer to the engine manufacturer's recommendation.
  7. Check the spark plug gap with a spark plug wire gauge. Never trust that the plug is properly gapped straight out of the box.
  8. Wipe a small amount of an antiseize compound on the plug threads. This will prevent thread galling, allow the plug to be torqued to consistent torque values and facilitate the next plug change.
  9. Install the new plug by hand. It is a good idea to make sure the threads are, in fact, smoothly engaging. With a tool, it can be all-too-easy to cross thread the hole. Should the plug begin to tighten prematurely, stop and investigate. This may signify a problem.
  10. Tighten it and reinstall the boot. Continue tightening until the crush gasket makes contact with the head. Get out a torque wrench and take the plug down to the manufacturer's recommended torque value. Wipe the ceramic part of the plug clean and reinstall the spark plug boot.

After installing a new spark plug, look at the old plug to get some idea of how the engine may be running. Wet, oily deposits suggest a problem — the engine may simply be overfilled with oil or it may be showing signs of wear. A new spark plug has sharp edges on the center electrode while a rounded center electrode means the plug is worn. Deposits, usually carbon, around the center electrode are normal, providing they are not excessive and are dry.

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