Spark Plug Thread Repair

Damaged spark plug threads in the cylinder head can be serviced using Rotunda Sav-A-Thread Spark Plug Kit 107-R0921 or equivalent.

The procedure involves cutting new threads in the spark plug hole. After completing rethreading operation, tapered sleeve will be installed in the cylinder head. The sleeve is threaded on the inside and outside. The outside threads into the cylinder head while the inside provides new threads for the spark plug.

The service is permanent and will have no effect on cylinder head or spark plug lift.

CAUTION: The cylinder head must be removed from engine before installing a tapersert. The service procedure includes a thread-cutting process which produces metal chips. Performing this procedure while the cylinder head is on the cylinder block will cause metal chips to fall into the cylinder. Once in the cylinder, these chips can damage the cylinder wall when the engine is started.





1. Throughly clean spark plug counterbore, seat and threads of all dirt or other foreign material.

NOTE: The use of power or air-driven tools is not approved for the installation of taperserts.

2. Start tap into spark plug hole, being careful to keep it properly aligned. As tap begins cutting new threads, apply aluminum cutting oil to tap.

Continue cutting threads and applying oil until stop ring bottoms against spark plug seat.

3. Remove tap. Remove all metal chips using compressed air.





4. Coat threads of mandrel with cutting oil. Thread tapersert onto mandrel until one thread of mandrel extends beyond tapersert.
5. Thread tapersert into tapped spark plug hole using a torque wrench. Continue tightening mandrel until torque wrench indicates 61 Nm (45 ft. lbs.).

NOTE: A properly installed tapersert will be flush to 1.0 mm (0.254 inch) below spark plug gasket seat.





6. To loosen mandrel for removal, hold mandrel stationary and turn mandrel body approximately one-half turn counterclockwise. Remove mandrel.