How to repair cross-threaded spark plugs on an aluminum motorcycle head

Written by c.l. rease
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How to repair cross-threaded spark plugs on an aluminum motorcycle head
Correctly installed spark plugs provide spark to the combustion chamber in a motorcycle engine. (motorcycle image by Greg Pickens from

Aluminium heads reduce the weight of your motorcycle engine. The properties that make aluminium lightweight create a problem for threads cut into the material. Cross-threading threaded fasteners occurs frequently in any aluminium material. The threads of a spark plug are harder than the female threads in the aluminium spark plug hole. Installing a spark plug off centre causes the spark plug threads to cut the female threads. Providing harder threads in an aluminium head reduces the chances of cross-threading.

Skill level:

Things you need

  • Thread insert kit
  • Drill
  • Heavy grease
  • Rag
  • Tee-handle tap wrench
  • Cutting fluid

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  1. 1

    Remove the cross-threaded spark plug from the aluminium motorcycle head.

  2. 2

    Secure the drill bit supplied with a thread insert kit into the chuck of a drill. Coat the end of the drill bit with heavy grease.

  3. 3

    Drill the damaged threads from the spark plug hole. Wipe the heavy grease from the end of the drill bit. Wipe the corner of a rag around the inside of the drilled hole.

  4. 4

    Insert the supplied tap into a tee-handle tap wrench. Apply cutting fluid to the tap threads.

  5. 5

    Put the end of the coated tap in the drilled hole. Turn the tee-handle tap wrench clockwise to cut threads into the aluminium motorcycle head. Reverse the direction of the tap to unthread it from the aluminium head.

  6. 6

    Secure a thread insert on the end of the supplied installation tool by turning it clockwise.

  7. 7

    Turn the installation tool handle clockwise until the top of the thread insert sits flush with the top of the aluminium head. Reverse the turning direction to remove the insert tool from the threaded insert.

  8. 8

    Remove excessive cutting fluid from the aluminium head by wiping the area around the insert with a rag.

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