Spark Plug Thread Repair

Special Tools:

CAUTION: The cylinder head must be removed from the engine before installing a tapersert. If this procedure is done with the cylinder head on the engine, the cylinder walls can be damaged by metal chips produced by the thread cutting process.

CAUTION: Do not use power or air-driven tools for installing taperserts.

NOTE: This repair is permanent and will have no effect on cylinder head or spark plug life.





1. Clean the spark plug seat and threads.





2. Start the tap into the spark plug hole, being careful to keep it properly aligned. As the tap begins to cut new threads, apply aluminum cutting oil.
3. Continue cutting the threads and applying oil until the stop ring bottoms against the spark plug seat.
4. Remove the tap and metal chips.





5. Coat the threads of the mandrel with cutting oil.
6. Thread the tapersert onto the mandrel until one thread of the mandrel extends beyond the tapersert.





7. NOTE: A properly installed tapersert will be either flush with or 1.0 mm (0.039 inch) below the spark plug gasket seat.

Tighten the tapersert into the spark plug hole.





8. Turn the mandrel body approximately one-half turn counterclockwise and remove.





9. Use the Feeler Gauge and a suitable straightedge to check for cylinder head flatness.